DescriptionCooke Radio Slide Rule, model 4139, 10" duplex, made by Keuffel & Esser Co., New York, no date, circa 1955-1956. Mahogany, machined and laminated, nickel metal brackets, with laminated white celluloid plastic ruled ("engine divided") scales in red and black; 12-1/8" overall length x 1-1/4" high x 1/4" thick. Inlaid edges. Indicator (cursor) is improved glass, no patent numbers or maker's markings on indicator. With: original reddish tan leather case with K&E logo blindstamped on flap tongue. Model is actually 4139S with the 'S" indicating it was with the leather case. Per 42nd edition catalog of 1954: Designed to facilitate the rapid solution of radio engineering problems as well as for general use. Serial number: 929804. Patent number markings: 2,500,460 [1950]; 2,168.056; 2,170,144; Pat. Pend. Scales- Front: LC, A, ,B, T, ST, S, D, DI Back: L, DF, CF, CIF, CI, C, D 2pi Dating of this rule is approximate based on the serial number and the following information. According to detail on the Keuffel & Esser slide rule reference website of Clark McCoy, the ST scale was on the first edition of the rule from 1943 to 1955 and replaced as SRT in the second edition in 1955-1962. The copy here has the inlaid edge known for at least the second edition, but it would seem that the use of the inlaid edge style preceded the second edition. The rule here was apparently made during a period when changes were underway in manufacturing methods. K&E had their main office and factories in Hoboken when this item was made.