In WA state supposedly they require all cars built after January 1st 1964 to have front belts atleast, even if the car never had them to begin with. Maybe someone from WA can chime in on how they can really enforce this, especially since some cars may lack the strength in normal attaching points since there was no federal requirement at that time. Modifying a car to accept belts wouldn't be done to those who show in concourse type classes, no matter the car make.
Here's something else interesting too, and I believe a possible factory screwup if they were required by 1966. Someone who goes to local cruises by me has a 1968 Mustang convertible without seatbelts he's owned since the early 1990s. He had the rug out of the car and there are no holes or even backing plates, or evidence there were backing plates under the car for them. The floor pans show no weld lines or other evidence they had been replaced sometime in the car's past, just the factory primer with the Highland Green overspray near the door jambs and surface rust in a few areas. If I hadn't seen it with my own eyes I would thought someone had removed them in the past and redid the floors and forgot to reinstall them. All I can think of is a possible factory screwup that went unnoticed somehow. That still doesn't make sense to me since I thought the backing plates were automatically welded on the floor at the factory, but if I hadn't seen and looked the car over when he had the rug out I would think he was lying or misinformed. If it is in fact a factory mistake it could've been a big liability one, and how the end of the line inspector would ever miss something like that! Strange.
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1968 Mustang coupe, Acapulco Blue, 289 2v, C-4, Power Steering.
Hopefully a 1969 convertible or Sportsroof (non Mach or Boss) for next Mustang project
Trying to find my father's 1973 Mustang Grande he bought brand new. 3F04F126773 last known registration and title was in New Jersey, 1982.
1965-1966 Mustangs, When did they start putting in seat belts? 
