My assumption based on looking at '73 cars, order sheets , and Gm practices on NON ordered cars is that the 400 4BBL and the 455 had the M21 as an "Option". If the car was NOT ordered it got an M20 ( which was also available on the 350 2BBL Formula and Esprit, but NOT a Esprit 400 or a base 400 2bbl) By '73 they were the same price but "ease of selling and availability " "probably" pushed the more common M20 through the assembly lines. This leads me to assume that base T/A with a std shift got an M20 . I am sure there are original cars out there to check on though..
HFR's pic confirms this .. " M20 STD on Trans Am "
What is a bit odd is that in '71 the M20 was 35E $ 202.92 ( Prices when DOWN?? , this is what I find odd..)
HFR's Former car was probably ORDERED with the "Optional" M21 So I am Not disagreeing
