If the cartridges in the photo match the chambers, it appears by Dixon's book to be either the 10,75x60R Grundig or 10,75x60R Mannlicher. These two are nearly identical with a 12,5mm base diameter and a 14mm rim. The 346 grain (22,5 gram) bullet weight was pretty common in German 10,75s.
Dixon mentions that the Grundig version was produced from near 1900 to about 1933 and was loaded by most of the German and Austrian cartridge manufacturers; it also appears in the 1911 Alfa catalog which may have information about bullet weight which Dixon does not list.
He shows the Mannlicher version as being produced by Roth with a timeline of 1905-1914 or so.
For the "usual" double rifle shooter this one would probably not have much appeal even though it looks like it is in great condition. For people looking for the unusual and unique in double rifles it would be very desirable. This rifle probably is one of a kind.
