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

Hi all,

it may be a strange question, but is there anything on a 66 Barracuda that officially makes it a Formula S ?
Is there anything on the fender tag, or VIN plate ?

Or is it just a case of if it was optioned with certain things it WAS a Formula S.

I ask because I have seen a couple of 1966 cars advertised as Formula S models, but they don't carry the badge on the fender, and I'm ot certain what to look for.

Thanks,

Peter