There are no major differences in the ’71 and ’72 Mach 1s externally.
The 1972 Mustang Mach 1 got the NASA-style scoop hood; argent silver or black lower body finish; color keyed front bumper, rear fender moldings, and sport mirrors; black honeycomb grille with integrated fog lamps; sport hubcaps with trim rings; a 302ci, 2-barrel, V8 engine; and a sport interior with high back bucket seats, electric clock, triple instrument pod, black and wood grain dash trim, and deluxe carpet and door panels. The 429CJ & SCJ engines were dropped from the lineup.
The 1972 Mustang Mach 1 also lost the pop-open gas cap (replaced with the crappy standard twist-on cap). Mach 1s were powered by the 302 Windsor in base trim but you could upgrade to one of three options in the 351 Cleveland V8 range (2 barrel, 4 barrel or the new for ’72 R-code 351HO).