8-Bores reasoning sounds right to me. A beavertail tends to put more force on the lug. If that lug ever moves the rib is popped. Many an old hammer gun without an efficient check hook developed this problem & they weren't made of Chrome-Moly. Every time they were opened the forend iron striking the bottom of the frame bar, gave the lug a "Lick". While the beavertail most likely was not as destructive as this, still a lot of those 21s were used in competition & fired a lot of rounds. I suspect this has a lot more to do with it than the steel alloy.
