What's behind the B Pillar post base?

I found where the current seat belt is bolted. It's closer to the front of the vehicle. Between the center of the B pillar and the rear of the front door jamb. (see pic).
The retractable (3 point) unit I have is larger and will not fit between the seat and B pillar; which is why I was going to install it where it was shown in the original picture.
Depending on how thick the B pillar sheet metal is, I might be able to squeeze it between the seat side and the B pillar (up higher). My son is the only one that drives the car, and the seat never gets adjusted.
thanks

Picture info: taken from the open passenger front door.