A K&E Plastic Slide Rule Timeline Study

A statistical approach to later-year Keuffel and Esser manufacturing dates

As described in our earlier post regarding the serial numbers found on K&E slide rules, Keuffel & Esser used a 6-digit serial number starting in 1922 to match up the slide and stock pieces of their wooden slide rules during manufacturing. This process continued through to the end of their wooden slide rule production in the early 1970s.

When K&E began making plastic slide rules in the 1930s they used a similar process. Their earliest plastic rules, 5-inch models called the “Ever-There” models, were made of “white xylonite” plastic. These rules, with model numbers of 4097, 4098, 4097A, etc., were made for roughly 20 years starting in about 1932. The xyolonite Ever-There slide rules were made from blanks of plastic and were constructed in much the same way as the wooden rules. The blanks were engraved with a dividing engine and cut into pieces for smoothing and final assembly; as with the wooden rules, serial numbers were put on the plastic pieces for future matching during the assembly process. The entire manufacturing of these plastic rules was performed at the K&E facilities in Hoboken, New Jersey, where the wooden slide rules were also made.

The Ever-There series eventually was supplemented by a new line of slide rules made of plastic that began in about 1948. The so-called “Doric” line contained both 5- and 10-inch models, with 7 models in all. The Doric model numbers were of the form 90xx, such as 9068, N9081-3, and so on. They were made in a similar fashion as the Ever-There rules, but of a different plastic, and were also made in Hoboken.¹

But by 1952 both the Ever-There and Doric lines were phased out, and a new line of plastic rules came into being. As the K&E wooden slide rule models had model numbers of the form 40xx (like 4053, 4081, etc.), their new plastic models starting in about 1952 were given numbers of the form 41xx.

The new slide rules were made using plastic molded parts, a technique that had been developed and was being used for K&E’s Leroy lettering products. The Doric slide rule line also tinkered with this technique for a while. Though assembled in Hoboken, the plastic molded parts for the new rules were manufactured by American Insulator Corp. in New Freedom, Pennsylvania, and serial numbers were applied to the slide rule parts at that plant.

The equipment for the 41xx line in Hoboken made its way to Connecticut to the Salisbury plant in about 1956-57, and the 41xx rules were assembled there, though the molded parts were still created — and serial numbers marked — in Pennsylvania. And, as we mentioned in our previous post on the wooden slide rules, K&E’s model numbers changed from the “41xx” format to the new “68-xxxx” format in 1962, which may have resulted in other changes to the serial numbering for the plastic rules. Further details about the construction process of the plastic K&E slide rules will be provided in a future post.

Interestingly, in 1969 all of the slide rule manufacturing equipment was moved from Salisbury (which was closed down and put up for sale) back to Hoboken. This move did not work out well for K&E and within a year they moved BACK to Salisbury (which was not sold yet) and re-started manufacturing at that location. One has to imagine what was the impact on slide rule production during that episode. But production was back on schedule by 1971 … and then closed down altogether in 1976 as the electronic calculators had taken over. A Deci-Lon was the last individual slide rule made by K&E in July 1976.

For our previous post on this subject, we had accumulated serial number information for over 1000 K&E slide rules with serial numbers; we’re up to about 1200 now, with about 550 of them being plastic models. So, with the above history in mind, and with catalog information and other published articles available, the three of us set out to see what we could learn about these serial numbers in relation to their dates of sale or manufacturing. Our best estimates for dating the plastic rules are presented at the end of this article, which we will continue to update and improve upon.

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The Plastic K&E Slide Rules

Many slide rule owners have fallen into the trap of dating the plastic K&E rules using the same McCoy curve that describes the wooden rules, even though McCoy states clearly that the plastic rules had their own numbering systems. (Gotta read the whole article, not just look at the pictures! Speaking from experience…) So, how can we sort this out? Our approach started out as the following:

• From the K&E catalogs and Clark McCoy’s research as described on his web site, we can see roughly the years during which a specific plastic model was sold. A nice compilation has also been created by Mike Frey. So, with no other information, we start by assuming that the start/stop years of production (Y1/Y2, say) for that model were on the time scale shown in the available catalogs.

• We then began to collect data of plastic slide rules to add to the lists from our own collections. We then assume that the numbering for each model started at 1 in year Y1 for that model, and we assign year Y2 to the rule with the highest serial number. We attempt to obtain enough samples of each model such that we can obtain a likely “highest” Serial Number for that model. We then can distribute the other samples linearly — as a first approximation — throughout this start/stop time period.

• And, of course, as other information becomes available to us that sheds new light on the above, we can adjust accordingly.

For slide rule models known to have been produced over several years, we would like to obtain a sample size of about N = Y2-Y1 to have a chance of obtaining at least a reasonable enough determination of the largest serial number for that model, assuming randomly picked serial numbers. For a fairly complete range, 2N or more would be better.

We knew it was quite possible that many plastic models with short life span were made in single or limited batches and then sold over several years until the supply was depleted. But unless we know this with fair certainty, we have left our approach intact. A good example is the Analon rules, which have been documented to all having been produced in 1967.² And we have found some slide rules, such as the Kissam Stadia Rule, which — for the data that have been collected thus far — appear to all have serial numbers within a very small range of large values, indicative both of having a limited production time and of likely not having their own serial number series.

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Test Case: The Ever-There Models

As an example of what we are seeing in the data, we have so far collected information for over 100 of the “Ever-There” slide rule models (4097, 4098, 4097A, etc.). Models 4097 and 4098 were sold from about 1931 to 1935. The others were sold from about 1936 to about 1952. We started out by making a plot of each model independently where we assume a Serial Number of “1” in the year it first appears in a K&E catalog, and place its largest acquired Serial Number in the year it last appears in a K&E catalog; the years of other examples of the model are distributed proportional to their serial number. Accordingly, we create the following single plot of all of the Ever-There rules for which we had serial numbers:

The lines drawn are for each of the individual models, according to our default approach mentioned above: using appropriate year limits given by the catalogs, we make one line from 1 to the highest 4097 serial number; one line from 1 to the highest 4098 serial number; one line from 1 to the highest 4097B, and so forth. Note that the K&E catalogs were only updated every two years or so. Thus, a ±1-year spread is not out of the question in our interpretation of start/end years of production on many of the slide rule models. The appearance of two distinct groupings with overlapping lines is suggestive of two separate series of serial numbers.

The 4097 and 4098 rules were introduced in the 1931 catalog. If the 4098 was made at about the same time as the 4097s, then it would be natural for it to have been made along the same assembly line with its serial numbers co-mingled with the 4097s.

In 1936 the 4097 was replaced by the 4097B and the 4098 was replaced by the 4097C. The 4097D (Beghin scales) and 4098A (pocket Mannheim) were also added to the “Ever-There” group that year. The 4097B/D and 4098A rules were sold until 1952-53, while the 4097C was discontinued prior to 1952. These newer models have serial numbers that overlap with the earlier models, indicating that they indeed had their own Series. Our present hypothesis — not too hard to imagine — is that the 4097/4098 models had a single set of serial numbers, and then the serial numbering was reset for the 4097B/C/D/4098A product line. Thus, a better model of the Serial Numbers might be something like the following, where the reset occurred in about 1935-36:

At first glance this does not appear to be too different from the previous image. But the ordering here assumes a linear progression of all of the serial numbers for the 4097/98 rules as a single series — not a set of independent series — and that a second continuous sequence of serial numbers was used for the 4097X/4098A rules. This picture of the numbering appears fairly consistent with the catalog dates for the individual models.

We will continue to learn and adjust the best “dates of production” that we can ascertain for each K&E model, in order to improve upon each curve and set of curves of this type for the plastic slide rules.

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Test Case: The Deci-Lon

We have examined over 100 samples of the popular 10-inch Deci-Lon slide rule, including at least one benchmark item. We know from K&E catalogs that Deci-Lon rules were produced from about 1962 until the end of K&E’s slide rule production in 1976. We have also done a fair bit of research on the construction issues associated with these rules.

Most 10-inch Deci-Lon slide rules have serial numbers in the range 10000 to 260000. However, we have also found three 10-inch Deci-Lons with serial numbers between 317400 and 318400, and two 10-inch Deci-Lons with serial numbers in the 900000s. Based upon other markings on these five rules we can determine the best dates for when they were likely produced — the 300000s in about 1962, and the 900000s in about 1969. Also taking into account the “benchmark” Deci-Lon for which we know the year in which it was received by the owner, we produced the plot below as a hypothetical production curve. A detailed report on how this was derived is forthcoming.

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The Model GP-12

We have acquired data for 36 specimens so far of the Model 68-1565 — the GP-12 — and their serial numbers range from 1802 to 37637. We have also found four GP-12s that have no serial number at all.

The GP-12 apparently had its own distinct serial number series. On the slide of the GP-12 one finds, “Copyright 1964”, which is not consistent with being within a main sequence of a 41xx production line. The GP-12 first appeared in the 1967 catalog; but then, the previous catalog came out in 1964. There are references to the GP-12 in books found online that were published in 1965-66. The GP-12 was last shown in the 1972 catalog, which was the last catalog with slide rules. It is of similar overall construction as the Analon, model 68-1400, which were reportedly all made in 1967.

If we use start/stop years of 1965/1975, respectively, we get the plot shown below. As for the examples with no serial numbers, perhaps they were early prototypes or a different special run.

Our ultimate goal is to generate plots such as those above that can account for all of the plastic K&E models, and hopefully uncover more literature information and more benchmark slide rules to help in the process of nailing down start/stop years and other distinguishing features.

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Dual 41xx Lines?

After studying the 41xx models and the similar rules sold under the 68-xxxx model numbers, the most plausible scenario to us is that the 41xx slide rules ran on a single production line from about 1952 to 1962 with their own serial number series. Then, in about 1962, when the slide rules had their model number changes, on-going production of some of these slide rules were made with a serial number reset. It is quite possible that the earliest such rules that were labeled 68-xxxx on their box were still made with the plastic molds that had the 41xx serial numbers built in to them, as many boxes labeled 68-xxxx contain slide rules labeled with the equivalent 41xx model numbers. A good example is the 4181-style slide rules, which are seen in both the 41xx sequence and in the 68-xxxx sequence shown in the image below.

The 41xx sequence also contains the Kissam slide rule model 4143 (68-1486), which are indicated in “green” in the above image. All 18 of the Kissam 4143 stadia slide rules documented so far have serial numbers between 394275 and 399568. If they follow the same numbering scheme as the other 41xx rules (a seemingly reasonable assumption) then these numbers are consistent with the rules being made in about 1960-61, and consistent with their first appearance in the 1962 catalog. All of the individual Kissam slide rules found so far are labeled 4143 on the rule, but they were only listed in the catalogs (1962 and later) with model number 68-1486.

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Production Overlap

Our benchmark Deci-Lon slide rule (serial number 225540) was obtained by its owner in 1971 when he entered West Point.³ We noticed, however, that if we asked what year would its serial number correspond to if it had been a wooden slide rule, then the answer was about 1970. Since we had the function coded up already, we quickly plotted the following in a single image:

The serial numbers of the Deci-Lons manufactured in Pennsylvania in about 1970 appear to overlap with a good fraction of the serial numbers given to the wooden slide rules being made in New Jersey in that year! We also see above that in the Analon production that occurred in 1967 (indicated above), all of the Analon serial numbers would have overlapped serial numbers of wooden slide rules made in or about that year. Such occurrences could certainly explain McCoy’s statement that perhaps “plastic and wood framed slide rules were co-mingled in batches” in the late 1960s. (See the McCoy web site.) We are finding such occurrences in several of the plastic slide rule lines where there appears to be serial number overlap with those of the wooden rules over short periods of time, but exact timing of such overlaps may require further study and verification.

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Preliminary Conclusions

So far we have been able to compile a relatively large list of K&E slide rules with serial numbers, and we continue to add to that list. With more data we are curious to see if any other patterns show up that might give further insights into K&E’s production methods.

We started out interested in the wooden rules as much as in plastic rules. We are still interested in patterns that might exist in the data for the wooden rules, but we also saw a unique opportunity to uncover new insights into the plastic rule production. Overall, we see so far that the range of serial numbers for plastic rules is typically between 1 and ~500000. There is no roll-over of the serial numbers as there is for the wooden rules, but we see a variety of distinct serial number lines in separate plastic models and model lines — Ever-There, Doric, and the 41xx models. For the ever popular 10-inch 68-1100 (Deci-Lon) and its 5-inch counterpart — the 68-1130 — as well as the GP-12 and Analon, each apparently had its own set of serial numbers. Interestingly, we see that the 10-inch Deci-Lons appear to have started at a serial number of about 10000. Also interesting is the fact that all 18 of the randomly selected Kissam 4143 stadia slide rules documented so far have serial numbers that fall in the narrow range of 394275 and 399568. And, as noted previously, documentation indicates that the Analon slide rules were all made in 1967. It has become clear that there were indeed several serial number “Series” used in the production of the plastic rules, and that several of these Series had short-term overlap with the serial numbers of the wooden slide rules.

Our best guess at this moment for the years corresponding to the various serial number series used on the K&E plastic slide rules is presented in the image below. All curves definitely should be considered preliminary, though several are fairly well understood. With more data perhaps we will see even more differentiation between the series, but our hope is that the curves will eventually provide collectors with a better indication of the year of production of their favorite K&E plastic slide rule.

The work thus far is based upon several studies of individual models, each with several trials (and errors) leading up to the present hypothetical set of curves. We look forward to being able to produce updates to these findings in future posts and articles, as well as to provide further details uncovered from our study.

If anyone has a K&E slide rule — wooden or plastic — with fairly accurate knowledge of its date/year of purchase, please let us know in the chat or comments section below so that perhaps we can add it to the list of benchmarks!

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1. For a good overview of the current understanding of the Doric rules, see Jay Ballauer’s web site and search for “Sidebar: The Doric Family of Rules”.

2. Joe Soper, K&E Salisbury Products Division Slide Rules, Oughtred Society Publication (2007), p78-79.

3. This refers to Jeff Ilseman’s Deci-Lon, documented on the ISRM web site at https://sliderulemuseum.com/KE_Standard.shtml.