You nailed it! I appreciate you big timeLooking at the mag catch, it is either a WELL or E&C, which is both an APS2 spec, which later would become the base for Novritsch's SSG96. Just to make sure, check the chamber and inner barrel. The barrel should have an unusual cuts which would be used to retain it to the chamber. It should have a bushing of some sort and a retaining clip.