Bores measure out .643. I would immediately think the bores have been gutted out but both chokes start at about. 3.5 inches from the muzzle ends.

Was the overbore a fox feature to make them light?

The serial number looks like a 1913 gun. The gun weighs in at 6lbs 2ozs which seems in line with other 20 ga sterlingwoths with 28" barrels.