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

Since the 70's. I bought my 68 from a guy, and I'm all but sure he did not change anything but the rear gears and torque convertor. Everything else was original and untouched. The auto Trans was fried when I got it. My brothers 67 Barracuda was bought from the original owner, and I know he did nothing but regular maintenance. I always thought it was funny that the 383 Formula S had a 120 MPH speedo, while the 273 2 barrel had a 150 mph speedo. We later converted the 67 to a 273 Commando and I know he had it up to 140 MPH.