Based on everything I have read and seen the last a nd only year for a 12 bolts rear in Second Generation Camaro's rear 1970. I am sure some more folks will chime in on this. I have not heard of any 1971's with 12 bolts but stranger things have happened.
That's what I thought but my memory has been shot lately.
It's been said that the 8.5" GM corperate 10 bolt was equal to if not better than the 12 bolt. GM sure did test that out with the big block cars from that era.