1983 was the first year. Even it's a later four-eye it's worth restoring too.
Use this info to verify it.
http://www.specialservicemustang.net/DSO.html http://www.specialservicemustang.net/Doortags.html http://www.specialservicemustang.net/Bucktags.html But sometimes you get a car were the buck tags were removed and door was replaced. I bought a '85 FHP like that. But it still had the 140 speedo and floor plan reinforcement along with a few other things, so sometimes you have to look for other clues.