You can order those two rings right from this website. It won't help you now - as they have to be shipped - but something to keep in mind for next time. Here is the link:
1992 Mercedes-Benz 300D 2.5 Sedan - Filters & Belts - Page 1

It's called an Oil Filter Housing Cover Stud O-Ring. It's a buck for 2 rings. Sadly those are not included in the oil filter kits. I found that out the hard way as well...

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)