It's common for European bores to be on the "tight" side and I'm not surprised that you measured the bores at .531".

The 13.5mm stamp was the bore diameter when the gun was proofed and they are the same today - 13.5/25.4=.531".

At one time I had a 12 bore chambered English shotgun with 13 bore barrels and have seen them with 14 bore barrels.