We are sure that this was in the early 1960s while they were working through old 4000 series stock.
Post sold a wide variety of slide rules, both under its own name and under the names of various manufacturers.
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.
All Post slide rules with "Made in USA" come from this period.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.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.