Nice job on the red delux belts. One of my blue delux retractor covers was cracked so I sent the pair to Ssnake-Oyl. In return, I got new labels, new webbing and everthing replated.
Rear seat belts were a federal requirement in 1966 (at least in California). I believe Ford put rear belts in all 1966 models to get a jump on the new regulations.
To recap for passenger cars (trucks had a different time line):
1964 and earlier - no seat belt requirements.
1965: front seat belts required, rear optional.
1966: front and rear seat belts required.
1967: front and rear seat belts required, front shoulder harness optional.
1968 and future; front and rear seat belts required, front shoulder harness required.
I don't know when rear shoulder harnesses were required. For a couple of years, some cars would not start unless the front belts were used, my 1973 Torino had an alarm that went off which was annoying but deactivated very easy.
Jim