John,
That is the way that I understand it.
Based on the powertrain usage chart in the 1973 Firebird Assembly Manual, the L75 455 was only available with the M21. Both the M20 and M21 were available with the L78 400. The M20 was limited to the SD-455 only, and that’s due to emissions certification testing.
I’ve been trying to clarify what occurred for a while. I haven’t located any bulletins that outline transmission usage. I do have PHS invoices that show Formula 400s invoiced for M20 and M21 transmissions. None of the invoices includes AC, so that couldn’t be a factor. And neither included G92.
I haven’t come across any L75 Trans Am invoices with M20.
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