The "captive" nuts on the rear of the drivers seat frame are replaceable. The nuts are attached to a sheet mrtal clip that slips over the sttel channel that is integral to the floor pan. I think I used a grinder to remove mine.....I surmise rust in this area is a common occurence and attribributal to poor maintenacne of drains from the hood hinge and sun roof.....does that sound accurate?

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Rob M
Norwich, VT USA
1980 240D Euro delivery 4 speed manual silver/blue
~160K miles