What's behind the B Pillar post base?

There are seat belts in the car already. Two per side. One lap and one shoulder belt.
The shoulder strap is bolted just above the rear doors while the lap belts are bolted further up on the floor. My son is at symphony practice now, but when he gets home, i'll verify the mounting location of the front lap belt.
I do believe it's new carpet. It was originally a green car with tan interior.