By 1975, all passenger vehicles would have had a catalytic converter by federal EPA law. That doesn't mean some states allowed vehicles without converters to still be sold. But what is more important is your original question. I'm not aware of any state that can prohibit you from buying any car you want to buy. The real question is whether you can get that car to be legally driven in that particular state. Certainly CA and NY or two REALLY strict states that could prevent you from successfully passing inspections for a number of different reasons. But if the car is equipped with the 'original' properly functioning catalytic converter or a car that has had the catalytic converter replaced by a CARB certified converter assembly, despite the year, then you are good. I say 'assembly' because in a state like CA, it's not enough to have just the converter replaced, the entire assembly with pipe, connectors etc all must be a 100% direct replacement to the original to qualify from a visual inspection alone. Then in the these same very strict states, your vehicle must also pass the emission test. For example, you could have the correct perfectly functioning original or CARB certified converter assembly but your carburetor could cause you to fail the emissions test. I hope this helps.
