1. the script heavily implies and the novel outright states that it is the human's arm that is cut off with ponda baba and the little rat like thing both getting cut in half.
2. when shooting the scene it was decided that it would be ponda baba's arm that gets cut, and when you look at the footage from the lost cut which is framed differently and you can see more of what is going on, it is clear that the human's arm doesn't get cut (you can see it as he falls back) and ponda's arm is seen on the floor having been chopped off when obi-wan swung at both of them.
3. for some reason it was decided not to use the footage of ponda's arm and to use the hairy hand instead the only reasons I can see for this are either it didn't look right because the flipper couldn't hold a gun or they wanted it to appear to be the man's arm and just modified the existing prop and shot it when they were shooting the extra cantina footage.
But regardless of what the intention was at the time I think the final edit of the film makes more sense it it is the human's arm, perhaps they didn't make it look entirely human as they were worried about getting a PG rating, but I've also read somewhere (maybe T-bone's site) the hairy arm was meant to belong to the rat like creature who was originally scripted to get killed in that fight.
now I've thought about it some more I can see it from both angles but I think the real problem with the flipper is the question "how would he be able to pull the trigger?"
I think for me I'm still just on the side of going with the human arm, but this is Adywan's edit so it's his choice.
Also Adywan it's really cool to be in the credits of star wars, thanks for that.