There is a round piece of rubber under the clothes hook on both sides of the cab. What is that?

 

Remove the rubber plugs and you'll see threads. This is where the optional shoulder straps attach.

Lap seats belts became standard equipment in 1967, but the shoulder straps were optional and were separate from the lap belts.

With shoulder straps, the lap belts have two buckles. One for the tongue of the lap belt, one for the tongue of the shoulder strap.

 

Thanks. I couldn't figure that out in 2 hours of Google. Now I can sleep.