wild guessm but probably part of the seat belt mounts.

could also be a support for the rear panel that goes in the so called seat area. I'd bet money its one of the two

 

Some bracket that were needed when a rear seat was installed?

 

Assume you mean the metal tabs attached to each side? The metal substructure that goes from the floor (behind the seats) up at a 45, to the rear hatch area rests against these 2 tabs. Right where there is a cutout for the shoulder belt to feely pass through. I imagine they provide a little additional support, but the main purpose is probably to help dampen vibration / noise of the substructure panels.