Depending on how charitably you define blade, I've seen locking scouts (I think Imperial, usually in the smaller size ranges) and there's obviously the ol' TL-29. I have a Crosman stockman with a lock on the clip point. It's probably a rebadge from one of the big old companies like Camilus or Imperial, and it's stamped USA. Nice little knife, has serrations on the sheepsfoot. My wife likes to take it as a food prep knife when we camp since it can slice bagels and stuff. They're not too hard to find on ebay. I could've swore Case made a lockback trapper, but I can't Google it up now.
EDIT: Google around has revealed a Camilus lockback trapper. Not seeing a lot of examples, and all of them are pretty spendy though.