O-Ring for front end of injector shaft

Hi Experts,
I have an oil leak in my 1984 240D where the injection pump attaches to the engine. I already have the gasket and the original 3 very fat and heavy MBZ manuals. I saw an exploded
view of the pump and see that it has an o-ring on the front end of the injector shaft where it passes into the engine. I forgot to order that. Does anyone know if this is a standard o-ring? Most likely it is metric and should it not conform to some spec for the hot and oily engine application?
I guess I should first remove the pump to inspect the o-ring and replace it only if damaged.
Is a gasket cement necessary for this job? The manuals are great on pictures and diagrams but very weak on good sets of instructions.

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