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1957 - Success of the Versalog prompts Hemmi to make pocket versions, the Post pocket Versatrig (1451) and Versalog (1461).

1970 - Post becomes Teledyne Post in 1970, slide rules sold under both the Post and Teledyne Post names.

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1932-1941 - Hemmi-manufactured rules have no date code and are marked "Made in Japan" 1941-1946 - WWII.

Post buys slide rules from Lawrence Engineering Service, Charvoz-Roos, Bruning and Gilson.

1963 - The change from state abbreviations to two letter codes (Colo. to IL) 1967 - The use of 5 digit Postal Zip Codes in USA addresses now mandantory. 1896 Charles Bruning leaves Post & Jacobi, and after a honeymoon in Europe, returns to the US and settles in NYC. He sells his blueprint business interests to George W. 1899 In April, Charles Bruning returns to NY and starts the Charles Bruning Co. The Fuller and Thacher's slide rules had their own serial number systems. There were logical groupings of numbers and dash numbers were used to depict length variations of a given rule type.

1983 - The use of Zip 4 Postal Zip Codes began used by USPS Many of the founders of United States firms producing slide rules had their roots as employees in other firms, just as modern entrepenuers break off from their employers and create their own start-up companies. Gustavson, in her article Necessity is the Mother of All Invention (Modern Reprographics, June 2000) is a good example. "N" was used as a prefix many times to denote some change in a given model.

Dates that a design was registered or patented only suggests an approximate year a model may have been first produced, as the model may have been distributed a year or more before issuance (patent pending). There were three rollovers occurring in 1943-1944, 1954, and 1962.

The patent may only refer to a certain combination of scales used over a thirty year time on different models. 1911 James Dieterich, who has worked his way up to blueprint foreman at B. Makepeace Co., goes west and lands a job as a salesman with the K&E branch in San Francisco. In general the production rate was about 70,000 rules per year until the 1950s. The 1960s had a production rate of about 100,000 per year for several years and tapering off in the 1970s ending at about 700000.

Post itself actually made no slide rules; even those rules that bore the Post brand name were manufactured by others.

At one time or another Post brand slide rules were made by Dennert & Pape, Nestler, Faber, Hemmi, Lawrence Engineering Service, Charvoz-Roos, Bruning and Gilson.

Dating a slide rule becomes difficult if a manufacturer did not imprinting a date code or serial number on the stock. This comprehensive research will enable you to find the period of manufacture for most every Dietzgen slide rule from 1887 through 1972.

Even then, some manufacturers recycled their serial numbers over time, and the stock could have been left on a shelf for a period of time before the slide rule was assembled and shipped. Combined Large 6.47MB Most Keufel & Esser slide rules had serial numbers, but unfortunately they reused their 6-digit serial numbers several times.

Catalogs are an excellent indication of when a model was produced or discontinued. This chart was developed by Ed Chamberlain to help determine the date of manufacture, but one must first zero in on which period the slide rule was made.

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