Found some markings
I found some markings under the forearm (I can't seem to upload pics)
There are three German proof marks, a crown over an S (Indicates Shotgun), a crown over a B and a crown over a U, both seen to indicate single proof, foreign arms.
There is a 78/41 marking inside a circle. I can't find a reference for that.
There is also a stamp that says 8,3mm. I figured that meant 8.3mm caliber, but the reference I found seems to indicate that might be the cartridge length. Which would make sense since it's a shotgun?
There is one more marking:
1.27
79
Not sure what that indicates.
So, this is a single shot, break open German shotgun of some sort...