It's just a nut on a plate rivetted to underneath. Just used nuts where I could get my hand behind whilst screwing the bolt in (battery box) but 2 man job for the other side of each seat and middle seat as I'll hold nut under car whilst the other screws the bolts down.So, is that a Rivnut, set into a plate, which is then pop riveted into place ?
If so ...... Rivnuts are available on the bay and the zone for not much money - and you wouldn't need the proper tool cos you've got access to both sides