03A2
I think we used a 12 guage stock on a 20 guage reciever before. The tapered stud coming out of the back end and the radial groove around the back of the reciever were the same, but we can't say for sure about the short tang on the trigger group, but if the stock is semi inletted there should be enough extra wood to make it work. The difference was in the outside circumference at the stock head and had to be pruned down and blended into the 20 guage frame. We always tried to slim them down and reshpe them anyway, so this was no big deal. Won't swear to any of this as it was a long time ago, and we could only afford to make do with what we had then. Don't be afraid to use some epoxy in there where it doesn't show.
Good luck'
D.B.
P.S. If you like, I can get out a couple guns and take the stocks off and do a rough measurement..... enough to know for sure.