74 duster seat-belt bolt not coming out

Is the slot for a thin wrench? Usually, the slot is to insert the sheet metal nut. Odds are it broke. If you can see how it broke, I've seen people take the piece of threaded thicker metal from behind and tap it with a sheet metal screw from the outside, right next to the bolt head and go for it from there.

Get a good view of what's going on back there. I don't think anything, outside of a drill fastener, cutting it open, or placing a wrench back there is going to get it out.