If you look carefully, I believe you can see his hand also before Darth Vader tells him he is his father. When he jumps down onto that little platform thing, and then shows Luke's face while he's talking, his arm (minus the hand) is across his body and at the bottom of the screen (if you look carefully) you can see the hand...It looks like they tried to make the hand grey to match his clothes in order to hide it.
If you look carefully when Luke clutches his wrist after Vader cuts off his hand, his right pointer finger pops up between his left thumb and left pointer finger (for only a few frames.)
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