Ted,

Toschi makes or made an 'under/over' such as you describe, not sure one of them ever made it to the states.

Handled a Cordy 20 O/U back in the 70's that felt at least as nice a wand as any ASEL 20 Beretta. At the time that Cordy brought many times what an ASEL 20 was going for. I really wanted to both shoot it and to get the calipers on it, but got to do neither. FWIW, the ASEL 20 Berettas' were/are the smallest framed 20 O/U's that I know of that could be called 'production' guns.

All above just middle of the night tangent thots; I do know that gauge wasn't mentioned. Anyway, more power to anyone wishing to take on the task of building a high grade double gun.