That is a cap for the oil pump drive bushing or 'thrust piece' as MB calls it. On a gas engine, the ignition distributor would go in that hole. The cap has an aluminum crush seal under it try tightening the cap. Should be a 22mm, maybe 24mm.

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Cars:
1993 400E
1993 300TE Wagon
1985 300D Turbodiesel w/ 688,000 Miles On Original Eng/Trans
1979 300SD
1977 Euro 6.9
1967 230S IMA Universal Wagon
100 Or So Parts Cars

You Cannot Change your Future Without Disturbing Your Present

It Looks A Whole Lot More Like It Did Today Than It Did Yesterday?