Just my spontaneous thoughts. Others may care to dissect the whole thing more slowly and carefully. Like the wear on various parts does not match up...
No organization was ever more picky about matching numbers on parts than the Germanmilitary. There is a comprehensive (and expensive) book of over 400 pages on nothing else but the marking of German military rifles and pistols. Recycled parts were typically renumbered to match the new weapon, the number of the donor weapon being lined out, not ground off. Newly assembled weapons were inspected and marked accordingly. Basically, if it wasn't stamped according to the regulations, to maintain traceability, it didn't officially exist. There was some rehashing of 98s during the Weimar Republic, but laminated stocks didn't appear until the late 30s, well into the Nazi era.
Whether army, navy, air force, police, SS, or any other organization that carried arms - they all had their well-recorded marking systems. And in that period, anyone creating a private mixmaster off their own bat would have been, literally, risking their neck. In fact, the most probable candidates for doing just that were the Jugoslav partisans, who recycled everything they could get their hands on, and were, of course, risking their necks. So it is prudent to assume that this is a post-war mixmaster, unless it can be proved otherwise.