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

6 leaf springs on both sides
stripes were optional, not standard and went fore/aft over the hood, roof, and trunk lid.
The exhaust has both a muffler and resonator
Until proven otherwise to me, disc brakes were not optional from the FACTORY in 1965. Dealer installed only. Factory option in 1966. 10" drums were standard on all V8's in 1966.
While you have to take my word for it. There is a certain '65 Formula S that I have seen the window sticker that showed disc brakes (the old lady original owner showed me the sticker after I told her that discs weren't a factory option in '65. She kept the sticker in the glove box). That was in 1990. Last I heard the car was in SoCal and I believe the owner may have been a member here, but that was around 20 years ago. I saw the car on Hwy 49 between Oakhurst and Mariposa at an antique store (could have been in Ahwahnee).