I'm involved in the replacement primer pump installation now on the 1984 300SD.
The instructions call for Loctite 638 retaining adhesive.
I have not taken the old pump off yet and cannot confirm fitment, but the new pump comes with an adapter ring. That is, the pump is a smaller diameter that screws into this adapter - the impression is that the adapter then screws into the car where the old pump fit.
The pump-to-adapter is sealed with a copper washer gasket.
Its the threads of the adapter-to-car interface where they want you to carefully clean and use the Loctite 638.
First question would be, is the adapter necessary for the 616/7 application?
If so, then the second question is how did you seal it?
Guess I'm good to go if the adapter ring is discarded.
Ken300D