Congratulations on the find, Dave.
I sort of figured there'd be other M31's to come along when I saw you over here at one of our western Montana shoots with the first one you bought a few years back.
As for the added weight of the solid rib barrels, it IS substantial. I've a 28" SR bbl. 12 ga. M31A from the late 40's for which I later found a plain 28" modified barrel. Weights are 2 lb. 2 oz. for the SR, and 1 lb. 12 oz. for the plain jane one - for a 6 oz. difference. The small rib profile can't account for all of that, and I suspect that the MWT is significantly more substantial than in the plain barrels based on my cursory measurements.) The SR 28" barrel here also has the same .031" constriction on a .731 nominal bore (Remington liked the number, I guess)and like yours, prints much tighter than one would expect at 40 yds.
A fun pump gun to shoot, and I hope you enjoy yours for a long time to come.
Robt.
