If my aging eyes are reading it right, the number on the flats close to the breech is "13", indicating a bore diameter when proofed between .710-.718". A 13/1 mark would indicate between .719-.728" and 12 would be from .729-.739". The 12-over-C in a diamond shows that it was intended to take 12 gauge cartridges. The original chamber length would almost certainly have been 2-1/2".
Was the 'cellophane test' done with the forend on or off? It makes a big difference and it should be checked with the forend off.
