Unfortunately the German Double Barrel, 12 gauge on left, and ??? on the right, has no markings other than the maker, (Immanuel Meffert, Suhl) the number 93, what appears to be a serial number, and a stylized letter that could be a K, or a combination of JK or IK. By the design it looks to have been made between 1870~1890, but that is just a guess, since I have not had hands on it.
No caliber markings whatsoever.
The owner did supply dimensions taken from fireformed cases, a Cerrosafe chamber cast, and a bore slug dimensions, and any help is greatly appreciated.
Just for fun, this is the stylized letter, and the might-be-a-serial number: