So... note to self... The center bumper sold on the repo market as C5ZZ-4730-B will only fit single exhaust cars,
yet the NOS center bumpers, (with part no. C5ZZ-4730-B) will only fit dual exhaust cars.
It appears that if I need a center bumper for a GT car, my only option is to buy NOS.
Plus, I'll add "snubber plate" to my list of GT car indicators.
Thanks Charles.