Post doesn't seem to have recovered from the disruptions of WWI.   They offered only  three "Frederick Post" brand slide rules in the 1921-22 catalog and I don't believe that two of them were actually availalbe for purchase. ( See Models 1444A-C. )   This is the one you could actually buy.

In gutter:  "FREDERICK POST CO.  MADE IN GERMANY" I have also seen a variant of this model with just "FREDERICK POST CO." in the gutter and a different table on the back.  (Columns: "Division of 90 deg.", Division of 100 deg.", etc.) Both variants were manufactured Koch, Huxhold & Hannemann.

This slide rule is listed in two sources: (1)  The 1921 and 1922 Post Catalogs  (The "Sectional View Showing Construction" appears to be derived from an illustration in the 1910 Post catalog and 1906-1919 Dennert & Pape catalogs.  I'm sure it was supplied to Post by a German manufacturer but I don't think it came from KHH.)    (2) "Instructions for Calculating Slide Rules" c1923-24