Okay guys I have more info from Mercedes after looking at some of their parts group diagrams. This part is found in Group 01 Subgroup 015 in the Benz computer. Engine is 616.912 - not sure if it applies to the 5-cylinder.

It's called the "Plane bearing shaft to oil pump".
The official part number is 615 181 0026.

Like you said, the dealership won't sell just the o-rings and it's $85 for the whole shaft plus a 2-week wait cause it has to come from Germany. Tiny little thing according to the diagram. Looks like maybe a 21mm bolt head that is as wide as it is long.
I will pop it out tonight and see if I can find matching o-rings at the local parts store to replace the brittle ones with. Otherwise I guess I wait! I'll let you all know how it comes out.

You guys are all fantastic help.

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Garrison R.
'83 300D