This is the rule that kept Post in the slide rule business.  In many persons' opinion it was the best general engineering slide rule ever made.  About one million were sold between 1951 and 1975.

The "Versalog" duplex slide rule was introduced in late 1950 or early 1951.  Many sources say 1950 but 1460 Versalog had not yet replaced duplex model 1462H Postrig in the July 1950 Frederick Post Price List.  Dual marked Versalogs [see below] are marked "Copyright 1951."  I'm going with the 1951 date.

Both Post and Hemmi said the Versalog was built for Post to Post's specifications but several early Versalog specimens are dual marked "Post 1460" and "Hemmi 258."   A few of these dual-marked rules are in Hemmi's standard aqua-green pasteboard cases labelled "Hemmi 258" and carrying a ¥2500 price sticker.  It certainly appears that Hemmi intended to sell these rules in Japan as Hemmi model 258 before changing their mind, recalling the rules, and adding Frederick Post markings.

Officially there were two versions of the Versalog:  the plain Versalog from 1951 to 1968 with 23 scales and the Versalog II, with 24 scales, from 1968 until the end of business.  Warren Salomon divides the "plain" Versalogs into two sub-models: "IA" from 1951 to 1960, with scales in navy and red only, and" IB" from 1960 to 1968, with the COS scale in green.  Versalog II re-arranged the existing scales and added an "A" scale for a total of 24.

The Frederick Post Company became "A Teledyne Company" in 1969, "Teledyne Frederick Post" in 1971, "Teledyne-Post" in 1972 and out of the slide rule business in 1975.  New catalog numbers were introduced in 1972 and the Versalog II became "Teledyne-Post Model 44CA-600."

The Post Company, itself, sometimes seemed confused about which model Versalog it was selling.  The Versalog II came out in November 1968 but the 1969 School Drafting Supplies Catalog and the 1971 Professional Drafting Room Equipment and Supplies Catalog didn't mention the Versalog II; they still described and showed model IB.  (The 1971 catalog incorrectly describes the Versalog IB as having only 21 scales.)   In describing the Overhead Projection Versalog, the 1972 Teledyne-Post Slide Rule catalog says "Each Overhead Slide rule duplicates exactly the scales on the slide rule being used by the class . . ."  but the Overhead Projection Versalog uses the "IB" scale set which had been replaced by the Versalog II scale set four years previously.
 

1955 Frederick Post General Catalog,  Pages 62-63

Versalog "IA"   Date code "EJ"  (October 1954)

Versalog "IB"  Date code  "NA"  (January 1963)

Versalog II Date code "UF"  (June 1970)