Seatbelt question: What year required shoulder belts?

I believe mid year 1968 is when shoulder belts were required for front outboard occupants. My 69 has spots for rear outboard shoulder belts. Slots in package tray sheetmetal. The rear shoulder belts appear to be a really rare option on 69.

Buy a set of wesco performance lift latch shoulder belts with upper sash and retractor mechanism and 12" long end with buckle. Forget about the OEM stuff. It was a pain in the ***.