I'm guessing that 13/1 was the original bore diameter; 12 was the diameter after a subsequent reproof. 12 is larger than 13/1. Highly unlikely to go from 12 to 13/1 (larger to smaller bore size).

I can't make out chamber length on the flats, although I think I see shot charge. If either or both are present, that would help estimate when the gun was reproofed.