1971 EV2 Dodge Demon "Sizzler"

I had a 71 Duster 318, auto car that came with an 8 3/4 so its possible thats the original rear. If nothing else has been changed on the car then why would somebody change just the rear? what bolt pattern is it? If its small I'd say its original. Sometimes build sheets were in the front seat backs and taped to the backside of the glovebox.