Zweiling refers to double barreled; Zwei is two. Buchsflinte probably refers to rifle shotgun because I believe that pflinte is an archaic term for a shotgun. In modern english a rifle shotgun combination is usually called a cape gun.
The proof mark is puzzling; looks British and yet the gun is certainly either Belgian or German. The circle 58 is also baffling to me because for those areas it should be metric and somewhere between 16 and 18 (mm) from your description. Possibly an unproofed german gun imported into England and proofed there.
Just my speculation anyhow.
cheers Doug
