What’s the exact process to identify a real 66 Formula S Barracuda?

In order to be a Formula S it also had to have the rallye suspension. Not everybody knew to check that box or to check the "S" package. My old '66 had every "S" piece, but was not an "S". I knew of another very similar to mine that was not an "S", either. Also, no such thing as a '68 383 Barracuda that was not an "S". Not until '69 when you could order a 'cuda 383...