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looking at some of the tiny details from the scrapped design, it looks like they originally wanted skullgrin's torso to pop up to a higher level, and come forward so you could have the skull crotch sticking forward.
I'm guessing the big compromise they had to make was to work with such a massive head they had to cut back on the transformation.
Had they gave him the robot head and made the skull be a snap on accessory it probably would have worked.