As Raimey accurately points out Boehler or as it is spelled really Böhler steel
comes from Austria.
It is not at all strange to have an Italian double with Böhler Antinit steel barrels.
My Beretta S3EL has them. I would think that Böhler supplied the steel and the
tubes were made in the Brescia region. In the case of Beretta I suppose they made
the barrels themselves. I'd add that that steel was used for the best range of guns.
As far as signor Belleri is concerned I have found this so far:
Church in Ponte Zanano:
Here there are several email adresses on the left side, and Antonio Belleri is named
at the bottom (Economia), where they list the different activities, as a maker of shotguns. Maybe someone there can help:
Valtrompia News JC
P.S.: Pictures of the gun would really help. jc