I know for at least 1977, a tire code on the broadcast sheet would be all encompassing for all GM suppliers, such as Goodyear, BFG, General, Firestone. As long as it was a "GM Spec radial tire". The size and if it was a raised white letter or not would mean a different code.
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1976 LeMans B09 Freeway Enforcer, 455/M40 Smokey
1977 Trans Am, 400/M21 Black/Gold Bandit. 44K actual miles
2017 Sierra SLT 1500 Z71 4X4 
2019 Canyon SLT Crew 4X4 
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