The fender tag and/or broadcast sheet will tell you for sure but there are some things that have to be there to be a Formula S. Some of these items could have been changed but originally they would have had:
Front sway bar with #892 and #893 torsion bars.
A cutout in the left side of the rear valance for the exhaust tip
6 leaf rear springs
150 speedometer with a thread-on cable
a 7 under the F on the fender tag
an E as the 5th digit of the VIN
14 X 5 1/2" wheels
Formula S emblems about a foot up from the bottom of the dog leg on the fenders.
Either a 273 4bbl or 383 4bbl engine.
A tach was optional but not available on a 6cyl car
