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

That's what I've been wondering is did some guy, or girl for that matter, walk into their friendly neighborhood Plymouth dealer and say I want a 383 4 speed Notchback and was told it needed to be an S and they said said OK, no, see ya and the salesman says wait wait wait. In the late Seventies I met a guy who worked at the Newark Plant who said "You wouldn't believe the cars we've built". He never told me what though. I think when the sales manager called the zone office and said the customer was going to walk that they would get something built for them.