The idea that it is a WW2 war trophy is not far fetched, at all. When there is a German/Austrian gun, under consideration, that has mounts but no scope, the most likely cause is what furhmann said. Many GI bringbacks were just shot with what ever fit into the chamber. There are several German/Austrian cartridges whose chamber will accept a 45-70 cartridge. A fired case would show which "base" the original cartridge is based on, as would a chamber cast, which at the same time would reveal case length.
Mike