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.
Post also sold branded slide rules from Richardson, Vicari, Boucher, Sexton, Halden, Chandler, Ritow and Winslow.
c1890 - Frederick Post founded in Chicago to manufacture and sell drafting, engineering and surveying supplies.
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.
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.
up to 1932 - Post slide rules were made by German manufacturers Dennert & Pape, Nestler and Faber.
1932 - Post began buying all of its straight slide rules from Hemmi Keisanjaku in Japan.