The Formula S was primarily a handling package, brought about by the Rallye Pack option from early 65. It consists of….
HD leaf springs, 6 leaves on the passenger side, 5 on the drivers side
HD “892/893” torsion bars, .870” dia
Front sway bar
14x5.5” steel rims
273 4bbl engine
Dash mounted Tach
Transverse stripes over the roof
High flow exhaust with no muffler, only the resonator
HD 10” drum brakes standard, disc optional in late 65
Formula S side badging on front fenders
As mentioned above, the fender tag for 66 will show a “32” under the “ab”, and a “7” under the “f”. In 65, the 273 4bbl engine designation would be “31” under the “ab”
That’s about it, although I maybe missing some small details.