1950 - Two character date codes appear (see Hemmi date codes).1951 - Hemmi drops the "Occupied" and marks slide rules "Made in Japan" or just "Japan".Post also sold branded slide rules from Richardson, Vicari, Boucher, Sexton, Halden, Chandler, Ritow and Winslow.See Who Actually Manufactured Frederick Post Brand Slide Rules, By Paul Ross c1890 - Frederick Post founded in Chicago to manufacture and sell drafting, engineering and surveying supplies.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.December, 1967 - Teledyne Acquires Frederick Post, slide rules sold under both the Post and Teledyne Post names. Still labeled as Post 1970 - Versalog II Date codes from 1970 are now labeled as Teledyne Post 1972 - The Frederick Post brand name disappeared and the Teledyne Post '44' numbering system was introduced. The date code (see Hemmi) is in the form 'HF' where the first letter identifies the year according to the system 1951 = B, 1952 = C, 1953 = D, etc., and the second letter identifies the month according to the scheme A = January, B = February, C = March, etc. The first letter is the year, as is listed in the following table.
1957 - Success of the Versalog prompts Hemmi to make pocket versions, the Post pocket Versatrig (1451) and Versalog (1461).1944 - The word 'POST' surrounded by an ellipse becomes the permanent logo.1946-1950 - Post resumes rebranding Hemmi slide rules, rules marked "Made in Occupied Japan".1931 to 1941 Slide rules manufactured by Hemmi Seisakusho.1932-1941 - Hemmi-manufactured rules have no date code and are marked "Made in Japan" 1942-1946 - WWII.Found on manuals and other slide rule ephemera: 1909 - Copyright Act on published works passed by Congress. 1951 - 3 digit Area Codes in Phone numbers implemented by AT&T (Developed in 1947). 1897 New York Blueprint Paper is founded by Charles Bruning in January. The common rules were produced in regular batches but the specialty rules did not sell as well and a given batch of rules might stretch over several years.COPYRIGHT XXXX and © XXXX 1943 - 2 digit Postal Zone codes to increase efficiency of delivery because of a wartime shortage of staff. 1963 - The use of 5 digit Postal Zip Codes in USA addresses adopted but not mandantory. 1898 After a fire, Bruning rebuilds the New York Blueprint Paper Co., only to close it and return to Chicago. It is not uncommon to find rules in the 1960s in 68 xxxx series packaging but 4000 series model numbers on the rules.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.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.Sometimes the copyright date in a manual is used to determine the approximate age, but one printing could span several decades. Look at catalog's in the museum library and visit Clark Mc Coy's: Serial numbers started in 1924 beginning with 0 and going through 999999 then rolling over to 0 again.1903 to c1917 Slide rules manufactured by Dennert and Pape c1920 to 1924 Slide rules manufactured by Nestler; Koch, Huxhold & Hannemann; and possibly Faber-Castell.1924 to 1930 Slide rules manufactured by Koch, Huxhold & Hannemann.Having contact with the original owner of a slide rule that remembers the year it was purchased helps on determining the age. In general it is safe to assume a linear production rate between the rollover dates particularly for the common types rules.