I am curious I thougth the MB Diesels sold in the US had 300, D, SD, TD or CD after the 300 on them.
Was your car origionally sold in Europe?

Also there is 2 types of Fuel Injection Pumps used on these Engines. The Fuel Supply Pumps are different on them and may be the threaded area size where the Primer Pump goes.
The "M" pump I believe has the Hand Primer close to the IP; while the other more common IP has the hand primer located further from the IP body and closer to the Driver Side Fender Well.
It might be better to look at the little Model plate that is on the Fuel Supply Pump and go to a Bosch Fuel Injection Shop and see which Hand Primer you are supposed to have.

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84 300D, 82 Volvo 244Gl Diesel