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

to my understanding, on the early cars VIN had no indication of "formula S"

fender tag was the identifier and corroborated by the build sheet.

obviously the motor, HD suspension & swaybar and specific parts of the interior group were part of the S package and would help in identifying a car. however, a lot can change in 50+ years... parts get swapped out, repaired, upgraded. so while it's helpful initially for a swift verification it's not the be all, end all of determining factors.

you want to be 100% sure? you need the trifecta: fender tag that matches the build sheet, build sheet that matches the vin. motor & transmission will not have vin numbers on them (that didn't start until 68) but should be date coded and within a reasonable time frame of the build. there are no body numbers on the early cars to match the vin.