Second ad Third Row Shoulder Mounts?

[64Bezer]

I have a 72 Suburban. It has front shoulder belts for the driver and passenger. The center front passenger and all second and third row seats are lap belts. My youngest is getting ready to move to a booster seat. Of course, you can't run a booster seat with just a lap belt. However, looking at the inside of the burb, there are tapped holes just below the headliner behind the second and third row seats. These appear to be original. They have the plastic plug caps that other front row shoulder belt holes sometimes have. I know that no suburbans of this era came with rear shoulder belts, but what are these holes for? I included a couple crappy pics of the locations of the plugs. Thanks to all that can help with this.
-Rob