The Atlas slide rule, with a spiral scale 50 feet long, was developed by Gilson much later than the Midget and Binary circular rules.   It first appeared in a Post catalog in 1960--which may be about when it was developed.  (If anyone knows when Gilson developed the Atlas rule please let me know.)   Gilson sold them to almost every American slide rule manufacturer/distributor.  Gilson rules never carried any indication of manufacturer or seller (or, for the Atlas, country of origin) 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).   The back of the Atlas was identical to the back of the Binary rule.

1960 Frederick Post General Catalog,  Pages 87

Atlas Circular Slide Rule Actual size: 8-5/16 inches diameter