I have a 1965 Mustang that I could legally drive without seatbelts in Massachusetts if I wanted to, as it was originally not equipped with safety belts.
Buying a car in the past when laws were different does not seem legally equivalent to buying a car today in a jurisdiction with different laws. It might be in some cases, but my impression is that the law is perfectly capable of distinguishing between those circumstances and it's possible to have them treated differently.
