Haha Lorne Peterson, nice. Peter Mayhew used to post at TF.N but I don't know if he does anymore.
It looks like this is what happened:
-In the script, the assailants are very exotically-described aliens, one being some rodent and another guy having a million eyes. It also describes the rodent being split in half down the middle, in addition to the severed arm, which it doesn't state who it belongs to.
-This was changed for the film because they probably couldn't afford such elaborate designs with the money they had to work with. I think a lot of the cantina costumes were recycled from other films and from pieces left laying around, especially in the original UK shoot (which is why so many inserts were done in pick-ups using much better costumes--most of the more memorable cantina monsters and aliens, featured in close-ups, came from this shoot). So they found an alien costume--one of the better ones from the UK shoot--that looked like a walrus and decided the guy with all the dialogue would just be a human with some prosthetics on his face, who would simply keel over. The walrus guy they decided is the one with his arm being lopped off, which they filmed in an insert, with a flipper.
-In editing, they probably thought either the flipper looked too silly and rubbery, or that it couldn't possibly hold and operate a pistol, or both, and so when it was decided to do cantina reshoots in LA that was a shot they wanted to redo, as it was pretty simple. This meant finding an alternate arm with a different, non-flipper design. The whole thing was supposed to be quick, almost impressionistic in the editing, so any mismatch is forgiveable--you can only really see the original flipper in wideshots if you freeze-frame, which wasn't possible at the time the film was made.
-Gary Kurtz selected the wolfman hand from the Rick Baker collection they used, probably because it was the best sculpt. They needed a hand, and it had to look alien and not just a normal hand. Apparently they had the entire costume there and tried to film a decapitation too, more in line with the graphic description in the script, but it was judged as too harsh, so you just had the wolf hand in the film.
-Flash forward 30 years and fans armed with VHS, Laserdisc, DVD and Blu-rays are freeze-framing through the sequence and noticing the reshoot/originalshoot discrepancy.