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

I'm pretty sure 66 up had 6 leaf springs on both sides. I know a "67" 4 speed Formula S 273 and "68" auto Formula S 383 both had 6 leaves on both sides. I never had a 65 Formula S.
Yes it did. I found that out with my Dart. It has the "Dart Rallye Package" option (code 368) with the HD leaf springs, sway bar, HD torsion bars, bigger 14" Blue Streak tires and 14x 5.5 wheels. THe 273-4 had to be ordered separately. It was included in the Formula S package.