No idea if this is relevant, but perhaps this 'shotgun' was meant principally for ball only, and has little or no choking? A reason for a the large muzzle aperture? Compared to a normal 12 g or 10 g gun meant for shot?

I am guessing it might have been meant for and used in Eastern Europe where sometimes I believe shotguns were the most that many could own (?).

Looks nice and unusual. Looking forward to hopefully some expert comments. BTW Welcome. I approved your application and a half hour later a post.