Thanks for posting the image, Tony.
TacDoc, the threaded hole for the fifth belt should be in the same raised channel that the seat legs bolt to; obviously it's in the middle and facing forward. Maybe your is covered with tape or paint?
The image Tony posted shows the belt coming through my seat cushion. The bottom end attaches to a bar; each end of the bar is held to the floor by the same bolts holding the rear seat legs to the floor. Many seat belt studies show that the belts should be horizontal; that seems to be common knowledge for the shoulder belts. Also applies to crotch belt, you want the support in the opposite direction of impact. Further, you want it tight before you hit, so having the crotch belt aimed down, such as it would be if you attach it to the stock threaded hole on the floor, means you have to slide forward before it tightens up; that would.... hurt.