Keuffel & Esser Duplex 4088

This duplex or double sided rule has patent dates June 5, 1900 and December 22, 1908. It was produced mainly from 1913 to 1939. It was available in four different sizes: 7 inches long (4088-1), 10 inches long (4088-2), 12 inches long (4088-3), and 20 inches long (4088-5). It was fabricated from wood covered with celluloid. The models produced before 1922 had 7 scales in the main side: K, A [S, T, CI] D, L, while the models produced after 1922 had 8 standard scales engraved on the main side: K, A [B, S, T, CI] D, L, all models had five scales at the back side: DF [CF, CIF, C] D. This rule was fabricated with both cursors, the frame-less and the wire-frame.

It is not easy to determine the dates the rules were produced, the manufacturer was not very consistent with serial numbers, some models have them, but others have none. In this collection the models 4088-2 and one of the 4088-3 were fabricated before 1922.

The exact original cost of these rules is not simple to find given because not all the precise fabrication year can be identified, but in the 1937 price catalog, the 4088-3 frameless cursor model had a cost of $9.35 ($195.34 in 2023 USD,  while the 4088-1 frameless cursor model had a cost of $8.50 ($177.58 in 2023 USD). In the 1938 price catalog, last one showing the 4088 models, the 4088-5 model had a cost of $23.70 ($505.67 in 2023 USD), the 4088-3 was marked $10.50 ($224 in 2023 USD), and the 4088-2 had a cost $10.25 ($218.70 in 2023 USD) .

References:

• McCoy, Clark: Keuffel and Esser Catalog Site. K&E Catalogs and Price Lists for Slide Rules.  http://www.mccoys-kecatalogs.com/KEmain.htm 

This website is the most extensive digitla file of Keuffel and Esser product catalogs and price lists (prices  shown here were taken from these lists)  from 1881 until 1972. You can find also user manuals for many slide rule models, as well as other interesting brochures and materials.

• Shawlee, Walter: Keuffel & Esser Co. Slide Rules. https://www.sphere.bc.ca/test/ke-sliderule.html 

This website gives a good overview of models, and other interesting information and links