Sorry Steve, I should have said "most" American cars had seat belts standard in 1963. My Dads 1963 Dodge "demo" had them standard, where his 1962 "demo" didn't.
In 1963, 23 states required them.
By 1964 nearly 1/2 the states required them.
In 1965, the U.S. Commerce Dept. issued its first seat belt standard.
ALL U.S. manufactures provided lap belts in front seats by then.
All "early" Mustangs were referred to as 64&1/2 cars, thanks to Fords "
PR". I believe the whole thing really got started with the 63&1/2 "Fast Back" roof lines, and the term just sort of stuck.
It does say 1965 on your title, doesn't it?