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

For all 273 Commando/Charger equipped cars - I always like to just look a the left rear valance for the big cut out. Even looked inside to see if it's been changed/repaired. Will look at 65 and 66 S fender tags later. My Feb built 4sp S car came with the 7 1/4 posi. The first 65 4sp had an 8 3/4 but not sure if had been changed out - got it in 1975.