1920 Single date

[Uncle Mike]

Can any one give me info on a 1920 dwm with a Mauser Arbiten proof(M over A) and only nitro proofs (crown over N).It also has an inventory number on the front grip strap beging with"O" and four numbers.My research has led me to believe that this is a parts gun made in the 20's by Mauser and issued to "ostfleet" but different sources give me conflicting answers. Mike C.