1946-1950 - Post resumes rebranding Hemmi slide rules, rules marked "Made in Occupied Japan".
1950 - Two character date codes appear (see Hemmi date codes).
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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.Fred Post goes to work at Eugene Dietzgen's company. Removal of the "N" prefix also indicated a variation in the model.The Bruning boys work for another blueprint company. 1893 Frederick Post, Charles Bruning, and Jacobi form their own blueprint company in Chicago under the name Post, Jacobi, & Bruning. 1962-1975 -The numbers were converted to a 68-xxxx number series.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.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.Post also sold branded slide rules from Richardson, Vicari, Boucher, Sexton, Halden, Chandler, Ritow and Winslow.All Post slide rules with "Made in USA" come from this period.1944 - The word 'POST' surrounded by an ellipse becomes the permanent logo.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.All these firms produced slide rules under their name as well as their primary business of drafting supplies: 1867 Keuffel & Esser, form an engineering supply house 1878 Eugene Dietzgen immigrates from Germany and settles in New York where he goes to work for Keuffel & Esser, now 11 years in business in the US. 1880 The Bruning family arrives from Nyborg, Denmark and settles in Cincinnati. 1885 Eugene Dietzgen starts a business in Chicago that later becomes the Eugene Dietzgen Co. Charles Bruning, his brother Jacob, and cousin Frederick Post arrive in Chicago. The use of the "N" prefix is not always consistent.