All '69 cars had the shoulder belts, just not the optional second seat belt receptical. First you had to put on your lap belt, then the shoulder harness. The deluxe center console had 4 slots, but the standard only had 2.
The hassle was what to do with the shoulder harness when leaving the car. My moms '69 Cutlass, which I drove every chance I could, required buckling and adjusting both belts.
The shoulder harness required the space of your fist between the belt and your chest. Too tight would limit your movement in the seat.
The shoulder belts in my Camaro look as if they have never been off their clips.
Good thing GM had colapsible steering columns.
I think '72 was the first year that shoulder and lap belts were combined into one unit.