Essentially, this is Megatron based upon the ACTUAL ROTF design. He has a crab claw right arm with the built-in gattling cannon and a normal sized right arm.

Leader Class Megatron was not based upon the finalised ROTF design. He was actually based on rather early concept art, which is why he has that tiny gimpy arm and the flip-out sword arm instead of the crab claw.

Hasbro got screwed by the movie people, again.