When I first got my car I noticed a bit of rattling out of the back seat when I'd hit even small bumps. I figured it's what's to be expected out of a car this age.
Recently I removed the rear seats to clean them and found that the bolts which hold the back cushion for the rear seat in place are missing (all of them).

Besides the rattling noise, that doesn't seem very safe to me. With only empty holes to go by, how would I figure out what the appropriate thread is? Is it likely to be worthwhile to try a home improvement store or should I just go to a Mercedes dealer?

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1982 240D automatic
Odometer quit at 307K...about 325K now