Wow is that rare.I'm guessing the same type used to mount the belts on the floor. If I can remember it tomorrow I'll check my '69 notchback parts car and let you know for sure. I've owned it for 3 or 4 years and just noticed last week it had the shoulder belts!
Dallas
Took a bolt out and measured it today. Looks like a regular seat belt bolt. It's 1 1/2" long total with non-threaded shoulder just under the head that's about 3/8" long. The threaded portion is over 1/2" long and the tip is about 1/4" long and doesn't have threads.
Dallas