It looks like it it a 32 gauge if I read the choke marking right. 12.9 mm in inches is 0.507874, 13.0mm in inches is 0.511811. A 32 bore is 0.526, a 28 bore is 0.550. and the misnamed .410, often thought of as .410 gauge is most nearly a 67 bore which is .411.
Easy way to check is drop a .410 shell in the chamber. If it is too small that rules out the .410. I suspect a 28 shell will be too large.
You still have very nice gun but in what to many is an odd gauge, somewhat limited in use due to small shot load. Fiocchi use to make 32 shells loaded with 1/2 ounce shot which is the standard .410 2 1/2" load. Who ever can afford that gun can afford to have a few cases of shells imported to shoot it. If I owned it it would be my opening day dove gun as long as shells last. A neat looking gun that would be a hoot to shoot.
