I think the alarm bells should have been ringing loud and long on this one regarding the possible non-originality of the barrels and chambering.
7x57 R is not really the way the poms would describe that cartridge IMO.
I wonder how many British hunting rounds were ever described as "rimmed" as most were called "flanged".
If the barrels and chambering were original Rigby I would have expected something like .275 Rigby Flanged" or similar.

Would be a nice little double though.