Panzer...I have a '91 300E. I can see through the hole to a cavity underneath (about 2" deep) which seems to be covered underneath by a fiberglass or thin metal bottom protection shield. Kind of like the shiled that guards the fuel pumps. My seats are powered. Already had them out and fixed, padded, and reinstalled. That's when I found the bolt would not tighten and secure and compared to the other holes which have a secured brass nut on the underside of the seat frame area.

Steve