Before chasing on that broken mixmaster in pawn shop, probably want to check this one out:
http://www.gunauction.com/buy/13417910
This one has a mismatched magazine floor. But that floor is not a moving part, so won't affect gun functioning.
Install a firing pin and safety lever, it will be fully functional. Aftermarket firing pins are available on gunbroker.com and ebay. Repro stock is flooding on ebay.
Of course, if this one goes too high, then return to pawn shop.