Manufactured by Gilson who sold them to almost every American slide rule manufacturer/distributor.  Rules never carried any indication of manufacturer or seller except "Made in US.A." so it's not clear that this should be identified as a"Frederick Post" slide rule.  But the cover on the instruction book was occasionally  printed with the distributor's name ("Frederick Post," "Dietzgen," Keuffel & Esser," etc).  Use your own judgement: Was Post trying to pass the Midget and Binary slide rules off as their own brand in the catalog pages below?

1949 Frederick Post General Catalog,  Pages 70-71

Midget Slide Rule Actual size: 4.25 inches diameter