It's my understanding that there was really no difference in the two other than the T-Bird had a black paint finish around the edges, no idea if during the production of the GT/CS they caputured the part before painting, I would assume so? But maybe they removed the paint (doubtful). Now days I think the reproductions are all painted because it's easier for CS owners to remove than it would be for T-Bird owners to paint them (could be wrong). As for NOS, I'd say if there are NOS CS frames out there without the paint that would be a good catch but the NOS for T-Birds with paint should be correct for the CS as well if you remove the paint, I highly doubt they had different part numbers.
This is all common sense speculation on my part and could mean nothing and I live a world away from trying to be Concours correct as I've probably identified in previous rants. I guess my point is that I would not pay a bunch of extra money for an NOS CS part without paint vs an NOS Tbird part with paint, they should be the same and removing the paint is simple, save the $ if that's what it comes down to.