This is a wild guess and I have no good sources more than a picture of a pewter town mark from Meissen (about 1690). I know well that Rosenberg suggested another mark with crossed swords over the letter M, but I think he is a bit vague.
There seems to be some traces of more marks below the crown, am I correct?
I will move it to the German section and let us see what they say about my suggestion.
Hope this helps.