HFR, do you have any additional evidence to share? Do any 4 spd base '73 Trans Am manifest build orders exist?
AMA specs can be misleading (or wrong) and I'm conflicted by other sources that point to the base Trans Am getting the M20 as the standard 4 spd in '73 with the M21 optional.
Here is the link to the listing I previously referenced, note that it is highlighted to indicate the M20 was "new or changed for 1973" which I interpret means the '72 Trans Am was different.
http://tran-zam.com/1973/Options.htm
My understanding is that in '71/'72, the M21 was not offered in any Firebird and the TAs did not use the M20, the only 4 spd for them was the M22. So what is shown in the linked chart is at least consistent in showing the M20 as listed for the TA and the listing of the optional M21 being "new or changed" and highlighting them as such.
Manifest records would surely be useful. Any original drivetrain '73 base TAs with documented M20 4 spd out there?