these period Pontiac have a degree of interchange, and when in doubt, a picture of the one you have with size and stud locations would really help.
There is generally at least one vendor at the Portland Swap meet (normally in 'Hall C' MAP LINK ) who has a literal bucket of these letters.
Come to think of it, thanks for the prodding - I'm going to make a note to keep an eye open to complete a pair of "LEMANS" letters I have currently on kijiji.
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1970 Formula 400 Carousel Red on black (std) interior "no drivetrain option" car (same base drivetrain as GTO) 1:411 1970 Firebird Formulas originally sold in Canada