Hi,
on both sides of the body the same steel part is fitted with 3 nuts each, see:
https://www.oudevolvo.nl/wp-content/...achterbank.jpg
The round hole in the middle of the steel part is for the pin / pivot point in the rear seat back (one side fix and other side spring loaded), the long hole aside in the steel part is for being able to press the spring loaded pin back (with a screwdriver or so) and release this pivot point of the rear seat back making it able to remove the rear seat back out of the car.
If complete this steel part is covered by a black plastic round cap.