Seatbelt question: What year required shoulder belts?

My '69 Barracuda notch didn't have any seat belts in front when I got it.
The headliner, which may be original (if not, it's at least 15 years old) has no holes for the shoulder mount. I can feel through it though where the mount should be, and it seems like it's there. Were shoulder belts mandatory in '69 in the US, or an option?