"So I imagine the small o-rings were not in place to begin with."

Interesting...I think those o rings are to keep the oil from running out of the filter when you stop the engine. The absence of which "should" have made your oil pressure "jumpy" Was this the case? Your gauge should be pegged when running and 1or 2 bars at idle.

The steel washer goes on the center post on top of the filter

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"