The +'s are indeed choke designations: mod/IM. 1.27 kg is the weight of the barrels. 75 indicates 3" chambers. 15.8 and 15.9 should be the choke diameters: .620 and .625. Working off the standard 20ga bore diameter of .615, that'd give you chokes of .005/.010, or significantly more open than the mod/IM indicated by the + marks. However, older Italian guns are often underbored, and if that gun has a bore diameter of more like .605 rather than .615, then mod/IM would be correct for the chokes.
The Italian Daly Empire Grades were indeed made by Beretta. You might even find their PB mark on the water table.
