That is beyond bizarre...Cut back the receiver to the locking lug??? The barrel itself looks nicely machined/turned...Can't see much of the front sight blade to tell if it is Luger or other...
OK...Here's my WAG...It was a DeWat, with a welded receiver/barrel...And someone cut out the welded section and fitted some kind of barrel...Can't be a rifle; not if it is 9mm...Might be a SMG barrel...Maybe...Or might be made from scratch...
I'd like to hear the story...