AFAIK, there is no such thing as a "drop-in" stock for SxS shotguns, unless (maybe) an exact replacement is purchased from the manufacturer - and even that would most likely need fitting of some kind.
The best you could hope for, ala "drop-in" would be to buy any BSS stock that came from another BSS of the same gauge, install it, and hope for a faily decent match.
You may get lucky, but sometimes, you get the bear.