The pieces of the puzzle are coming together albeit slowly. It seems there was a shift in the types of frogs used by the Wehrmacht circa 1940 which included not only the Luftwaffe, but the German Navy as well. Like most things it probably did not happen exactly at the same time or date for all makers, but sometime during the year or even beyond for a short while.
To the far left a 1939 dated Luftwaffe marked frog by E.O G�tze & Sohn. A black 1939 dated Kriegsmarine marked frog by Franz Brehme - Walsrode (not pictured) is basically the same other than than being from a different maker. This (AC 311) style frog has twin piercings to accommodate the tie strap which is a little more complicated to manufacture.
To its right is the more simplified (AC 310) style Kriegsmarine frog lacking the piercings also from Franz Brehme - Walsrode, but dated 1940.
Posted to its right is one of the seemingly much scarcer post 1940 AC 311 style frogs. With this example by bmn - Bottcher & Renner N�nberg-Ost dated 1943. Between 1940 and 1943 for most practical purposes the manufacture of the AC 311 style frogs seems to have been in hiatus. (As an aside, compared to some of its contemporaries the frog seems to be surprisingly well made.)
And to the far right a late no maker AC 311 style frog. Made from what seems to be less than top quality leather it probably was the best that the maker could find at the time. Another AC 311 style frog which does not seem to be particularly common. FP
