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.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 also sold branded slide rules from Richardson, Vicari, Boucher, Sexton, Halden, Chandler, Ritow and Winslow.
Post buys slide rules from Lawrence Engineering Service, Charvoz-Roos, Bruning and Gilson.
All Post slide rules with "Made in USA" come from this period.
1944 - The word 'POST' surrounded by an ellipse becomes the permanent logo.
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.
Fred Post goes to work at Eugene Dietzgen's company. Removal of the "N" prefix also indicated a variation in the model.