I had a post last week about a gun I just bought that I was told by the seller it was a 12 gauge. A 12 gauge shell will fit in it. However, it has the same markings as yours, that being 15 and 14. I was told it is a 15 gauge with 14 bore at the muzzle. If it accepts a 12 gauge shell, the chambers have been reamed out quite a bit.