Actually it is just as the harpu00 has said. above the Differential Mount sanwitched between under chassis sheetmetal and the bottom of the Trunk metal is a flat iron Bar. I believe there are threaded holes in the Bar.
It is possible that you could install a Helicoil into the stripped hole in the Bar.

However, the reason I can see the Bar on my car is because part of it is rusted through. I do not think missing one of 3 Bolts will cause an issue.

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84 300D, 82 Volvo 244Gl Diesel