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

The Formula S package was available in all three 1967-69 body styles. The only drive train option that was not available in the convertible was the '69 'cuda 440. 340, 383, and 440 ALL required the suspension package. 383 in '67 and '68 required the Formula S package ('cuda 383 was also available in '69). 340 in '68 required the "S" package. '69 340 was available in Formula S, 'cuda, and plain, but still came with the suspension package.