The Midget Circular Slide Rule appeared in the Frederick Post Catalogs and Price lists as Catalog Number 1439D in 1922, as Catalog Number 1463 in 1936-1942 and 1949-66, and as Catalog Number 44EA-630 in 1972 until the end of business.
The Midget Circular Slide Rule was manufactured by Gilson and sold, at one time or another, by most American engineering suppliers. Gilson never put their own name or the reseller's name on it's slide rules but sometimes they did put the reseller's name on the instruction booklet. Therefore it's not clear that this rule should be classified as a "Post" slide rule since the only appearance of the Post name may have been on the instruction booklet.
Above scan is from the 1922 Frederick Post Catalog.