As RM suggested, it is most likely a later Cowboy Companion stock. It could also be a stock off a Keng's sold rifle as well, as they also filled in the bayo groove on some models and also had a rear mount scope.
Also keep in mind, that Navy Arms would make any configuration of SKS that was requested, so it could really be off anything.
As for the magazine well, I agree with LC that it doesn't look like anything special. Just a rougher finished version. Both the Polytech Hunter 5 round mag and the aftermarket flat Chinese 5 round mag, did not require any modification to the stock.