8.15 K+E Mystery Rules Update

NOTE: This update has been updated since its original post. The latest update was on 2024 Mar 22.

In an earlier vignette called A K+E Rule of Mystery, a slide rule manufactured by Keuffel and Esser without a model number is discussed. As mentioned at that time, there apparently was a couple of models of slide rules that occasionally surface which do not have model numbers. Since the writing of that earlier vignette I have come across a few more of these, and the variations in these samples, that I and others have collected, indeed seem to keep the mystery alive.

Keuffel and Esser, Co., USA, was the major producer of slide rules in the U.S.A. Starting their own slide rule production in 1891, K&E manufactured somewhere between three and four million slide rules and, according to their catalogs, had over 120 different models and 2-3 dozen special production slide rules. However, a handful of K&E slide rules were manufactured and distributed that do not appear in any of the K&E Catalogs. Although stamped as being manufactured by K&E, and possessing proper K&E serial numbers, they do not have a model number. The Clark McCoy web site identifies these rules as the Mystery K&E Mannheim Slide Rules. He identifies three types, which I will call Type 1, Type 2, and Type 3, distinguished by their scale sets. His site shows two examples of Type 1, and one each of Type 2 and Type 3. The Type 1 and 2 rules both have 10-inch scale lengths, while the Type 3 is a more rare 20-inch slide rule. The scale sets are:

--   Type 1:   in /| A DF [ CF CI C ] D K || cm    ;   [ S B L T ]   
--   Type 2:   in ||    A [  B CI C ] D K || cm    ;    [ S L T ]   
--   Type 3:   in /|    A [  B CI C ] D L || K     ;    [ S A T ]   

The scale arrangement of Type 1 is only found on one other K&E slide rule – the 4097D. The scale set for Type 2 is the same as is found on the 4053-3 slide rules. The scale set for the rare Type 3 appears to be a “one-off” arrangement.

Type 1

In our earlier vignette, an example of a slide rule akin to McCoy’s Type 1 is discussed. Its serial number is 471354. Since writing that story in the summer of 2021, a second Type 1 “Mystery Rule” was obtained from a seller in South Carolina in November 2021. It was in better shape than our slide rule from Saginaw, Michigan, and has a slightly earlier serial number of 434918. Then, in April 2023, a third Type 1 was was found for sale in Illinois. Its serial number is 434587, just 331 slide rules earlier than the South Carolina item.

On his web site, McCoy shows two such rules, Specimen 1 and Specimen 2, both of which have serial numbers (632200 and 848106, respectively) greater than the ones in the present collection. One of his slide rules (Specimen 2) has “Engineering School” labels on the back side from the University of Washington, while our examples to date have not. Also, his Specimen 2 has a slide and stock of equivalent length, whereas his Specimen 1 and our three examples all have a slide that extends about 5 mm beyond the stock. The Specimen 1 type of stock/slide arrangement is familiar from the early Dietzgen slide rules. (See, for example, this Dietzgen Rule.) So far our early examples of Mystery rules in the Collection have been of Type 1, similar to Specimen 1.

But another feature to note is the logo on these Type 1 rules. In both specimens on McCoy’s site the logo found on the right-hand end of the slide is “K&ECO.”. This is also present on our first Mystery Rule with serial number 471354. However, our specimens with earlier serial numbers have “K+E” as their logos. (See image below.)

Mystery K&E Mannheim Slide Rule, Type 1; S/N = 434587.
Mystery K&E Mannheim Slide Rule, Type 1; S/N = 434587.

Type 2

More recently, in July 2023, one of the “Type 2” Mystery rules described by McCoy surfaced. His Type 2 – Specimen 3 – is also a K&E rule that has no model number; it has serial number 529602. Our recently acquired Type 2, coming from Florida, has a serial number of 291505. The Type 2 rules have a scale set of the Model 4053-3, different from the scales of Type 1, but again laid out on a different size stock. In both McCoy’s case and for our new example, the slide rule stock and slide are both of equal length – 29.5 cm for our rule. The Type 2 rules have patent information, Keuffel and Esser Co NY, and Made in U.S.A printed in red on the front of the rule, while the Type 1 rules have no such information on the front. And, both McCoy’s and our Type 2 rule have a University of Washington College of Engineering label on the back of the slide rule.

Mystery K&E Mannheim Slide Rule, Type 2, from late 1929; S/N = 291505.
Mystery K&E Mannheim Slide Rule, Type 2, from late 1929; S/N = 291505.

The text information on the back of the slide rules appears to vary from rule to rule. Our Type 1 rules so far all have the same text layout, which describes how to use the slide rule and how to keep track of decimal points. This is seen on McCoy’s Specimen 1. But both our Type 2 rule and McCoy’s, on the other hand, have tables of constants, formulas, and other useful information. McCoy’s Type 1 / Specimen 2 also has this text.

But speaking of text on the back, there is another slight difference between our new Type 2 rule and that on McCoy’s site. For our new rule, the College of Engineering label for the University of Washington is located in the middle of the text on the back, rather than on the right-hand end as in McCoy’s case. And the text of our new rule is laid out in various panels which are printed in a “landscape” format on the back. In McCoy’s Specimen 3, the same panels are laid out in a “portrait” format, where the user might orientate the rule up and down in order to read the text.

And then there’s the cursor. All of the previous Type 1 and Type 2 Mystery rules mentioned above have the “improved metal rimmed cursor”, introduced in 1936. But our new Type 2 rule has an earlier “all-glass cursor”, first introduced in 1915.

It is not clear to me yet if these new examples in the Collection add to the understanding of these Mystery Rules or just add to the mystery. So far these data continue to support the conjecture that the Mystery slide rules were special production runs, perhaps to be sold at university bookstores or for distribution from university departments. Until now there have been reports of examples from 1931 through about 1950. However, with our new find, we have a Type 2 Mystery rule that was likely manufactured in late 1929.

In order to better document what is known about them I submit the following table of Mystery slide rules comprised of both our Collection’s samples as well as others I have found on the McCoy web site and on the sites of Frey, Ramirez, Ballauer, the Paul Tarantolo Collection at the Oughtred Society, and the ISRM. These web sites were chosen as they have fairly complete data for their samples. Note that text found on the Tarantolo site mentions the University of Wisconsin as being on certain slide rule logos, but all of the corresponding images verify that the logos are indeed for the University of Washington. We can say the following based upon our set of examples:

  • The Mystery rules began to appear as early as 1929, and were produced during the next 20 years.
  • The panels of text on the U. Washington Type 2 rules appear to have changed from landscape to portrait layout in 1931.
  • All slide rules documented in the list with university logos appear to be for the University of Washington.
  • The slide rules with University of Washington labels all have equal length stock and slide, true for Mystery Type 1 and Mystery Type 2.
  • The logo varied between “K&ECO.” and “K+E” throughout the span of production.

My web search for other Mystery rules was not at all exhaustive. We know that these rules are not common, but they are not extremely rare, either. Perhaps others with similar examples that can add to the story would like to contribute to the list below? If so, please contact me; perhaps we can solve the mystery together, once and for all …

Mystery K&E Mannheim Slide Rule Comparison Table

Source S/N Year Type Logo Patent Case Cursor Slide Table Layout School Link
MSy 291505 1929 2 none c: 8.17.15; f: June 5, 1900 leather a-g flush L Was-c MJS_T2-1
PTa 402458 1931 2 none c: 8.17.15; f: June 5, 1900 a-g flush L Was-c PTa-1
PTa 402655 1931 2 none c: 8.17.15; f: June 5, 1900 a-g flush P Was-r PTa-2
PTa 403313 1931 1 K&ECO. c: 8.17.15 a-g ext. L none PTa-3
MFr 434542 1933 1 K+E c: 2,086,502 m-r ext. L none MFR_T1-1
MSy 434587 1933 1 K+E c: 2,086,502 leather, belt clip m-r ext. L none MJS_T1-3
MSy 434918 1933 1 K+E c: 2,086,502 black cardboard m-r ext. L none MJS_T1-2
MRa 435059 1933 1 K+E c: ? m-r ext. L none MRa_T1-1
PTa 460528 1934 3 none f: June 5, 1900 m-r flush L none PTa-4
PTa 468962 1934 1 K&ECO. m-r ext. L none PTa-5
MSy 471354 1934 1 K&ECO. leather, belt loop m-r ext. L none MJS_T1-1
MSy 529600 1936 2 none f: 1,934,232 leather m-r flush P Was-r MJS_T2-2
CMc 529602 1936 2 none f: 1,934,232 m-r flush P Was-r CMC_T2-1
MKo 554194 1936 1 K+E c: ? m-r ext. L none ISRM_T1-1
PTa 564336 1937 1 K&ECO. m-r ext. L none PTa-6
CMc 632200 1937 1 K&ECO. c: 2,086,502 m-r ext. L none CMC_T1-1
JBa 632308 1937 1 K&ECO. m-r ext. L none JBa
PTa 692285 1940 1 K&ECO. m-r ext. L none PTa-7
PTa 774832 1941 3 none m-r flush L none PTa-8
MRa 790092 1940 1 K&ECO. c: ? m-r flush P Was-r MRa_T1-2
PTa 790347 1941 2 K&ECO. m-r flush P Was-r PTa-9
CMc 848106 1941 1 K&ECO. c: 2,086,502 m-r flush P Was-r CMC_T1-2
PTa 877545 1942 2 K&ECO. m-r flush P Was-r PTa-10
PTa 877776 1942 1 K&ECO. m-r flush L none PTa-11
PTa 924545 1943 3 none c: 1,934,232 m-r flush L none PTa-12
PTa 358978 1948 2 K+E m-r flush L none PTa-13
PTa 434672 1949 1 K+E m-r ext. L none PTa-14
PTa 552503 1950 1 K+E m-r ext. L none PTa-15


  • Source – MSy = Syphers; CMc = McCoy; MRa = Miguel Ramirez; JBa = Jay Ballauer; PTa = Paul Tarantolo; MKo = Int’l. SR Museum
  • Year – obtained from McCoy’s Plots/Tables of K&E serial numbers
  • Patent (see Notes below) – c: = on cursor; f: = on front
  • Cursor – m-r = improved metal-rimmed; a-g = all-glass
  • Slide – whether the slide is flush with the stock, or is extended
  • Table Layout – L = landscape; P = portrait
  • School – U. Washington: Was-r = label on right end; Was-c = label centered


  • The Improved Metal-Rimmed Cursor was introduced in the 1936 K&E catalog.
  • Patent no. 0,651,142 – Jun 5, 1900; alignment screws.
  • Patent no. 1,150,771 – Aug 17, 1915; Slide Rule Runner – all-glass.
  • Patent no. 1,934,232 – Nov 7, 1933; method and means of securing slide rule elements together.
  • Patent no. 2,086,502 – Jul 6, 1937, Runner for Slide Rule – improved metal-rimmed.

For a wonderful look at some Mystery rules, one should visit The Paul Tarantolo Collection at the Oughtred Society web site, where his 17 K&E Mystery slide rules are displayed.