The M1916 is a six-shot copy of the Smith & Wesson top-break revolvers, made for the Italian Army from 1914 to 1918 (says Hogg).
The Central Firearms Registry thinks this is a 45 of some type, but 45 Colt ammo does not fit in the cylinder. The barrel lands pass a 0.418 pin gauge but not a 0.419. So one would expect the bullet to be .420 or so. The cylinder front diameter is 0.439. Cylinder is about 20mm long.
Based on this, I'm going to go with the caliber being 10,35 (10,40) Italian Ordnance. Everything seems to fit. And it matches what Hogg says.