If allpar is correct,and the switch was in April, that could mean more than a few forged are out there. When was start/stop for model year production back then? October/ July?
First week of August was the start of production. End was early July. My '72 Challenger has a build date of 8/17/71, and a lot of '71 specific parts too. They just used what they had left over until it was gone, which is also why a lot of A-bodies ended up with rim-blow steering wheels in '71, they were going out of production for '72.