71 Demon What is this trunk/seat belt hole for?

Hey Guys,

Pulled the rubber mat out of my 71 Demon yesterday in preparation for carpet. In the back on the passenger side where the back seat belt bolts to the trunk floor, is a hole with a bolt in it. The bolt does not attached to anything. It's just stuck in the hole. Does anyone know what this hole is for? Please see attached pic. It's right by the seat belt bolt.

Thanks !