I'm not aware on any facts regarding these control marks. We find them, as said, on several makers' pins, and also in the eyelet e.g. of "R" crosses. I don't think they were meant for the GOK, but rather something workshop internal - which explaines only some makers have them. Imagine there were five guys assembling the EK parts at Wagner, and one being a total bumbler...
No, we don't. That's why I said "I believe".
You have an EK1, and I had one in hands yesterday. Finding the mark on 2nd classes, however, is indeed a bit odd but doesn't change much of my thoughts.