This demonstrates the lack of continuity, where it can't possibly be Leavelle's forearm.
So, you see where Leavelle's left shoulder is, which is where his upper arm is. We can see the line of his upper arm coming down. That's got to line up with the forearm. But, the outer edge of the forearm is nowhere near it. It's just hanging there, totally severed from him. The only thing that makes it appear to be Leavelle's is the fact that it's cream-colored, like the rest of his jacket. Oswald is wearing a black sweater, so it can't be any part of him. So, who is it part of? NOBODY! It's just an artifact that they added to the film to make it look like Leavelle was walking along with his hand in Oswald's pants.