What "makes" a 66 Barracuda a Formula S ??

Not really sure how to tell the difference on the 14 x 4.5" wheels, but both my old '66 and old '67 Barracudas had K-H discs and the 4.5" wide rims. Then again, both cars came to me that way so I never had to search anything out.
I don't know either, but, I've run across some that rubbed.