We do have a Service manual for the OM 636/621 engines. Its a very early copy. It states there we 5 different versions of pre-combustion chambers up until the time the manual was printed. I have found a number of changes with parts on the engine that are not in the manual.
I was wondering if there were more versions that did away with the ball pin as there are none in any of the chambers. I was hoping to prove it was not due to pin burn up!