What’s the exact process to identify a real 66 Formula S Barracuda?

14 x 5.5 rims. Discs were optional. Not all '66 Commando cars had the 8-3/4, including four speed cars. ALL '66 Barracuda VINs start "BP" and ALL 273 cars (2 or 4 barrel) have the "D" in the fifth character of the VIN. So, as long as the VIN says 273, it is of no further help. As stated above you need the fender tag with the proper codes, but the only way to match the fender tag to the VIN is with the broadcast sheet.