Hmmmm, maybe the 4cyl 1979 had a turbo, but can't remember for sure and not sure what model it came it.
maybe the indy 500 package??
my last guess is 1979
A senior at my High School had a 79 Mustang Cobra with the 2.3 Turbo. He would regularly smoke the rear tires much to the dismay of some of the Teachers. Was also available on the Pace Car. The Mustang GT Moniker didn't reappear till 82. 79 to 81 the top model was called the Cobra. 1979 is correct and Trevor got it.