I don't pretend to know more about the history of this gun than you or your father, but it shows signs of of having been buried or otherwise hidden under harsh conditions to avoid having to turn it over to postwar authorities for destruction. This was often done and is a lesson about the futility of trying to confiscate citizens guns. After the war, many such guns came to light, often to trade for food , and quick cleaning methods didn't always kill the rust which could continue during years of further storage.
Mike
