I have just successfully removed the ocular sunshade without damage and there is no serial there either. So there is no serial at all on this scope.
Now I can see that the ocular seems shorter than other ZF41 ocular pieces. I saw that on another "M" marked ZF41. Would there be a link between this "M" and the shorter ocular ? When I look through the scope, it seems this ZF41 was made for a shorter space between the ocular and the eye.
Here are some pictures of another "M" marked ZF41 (not from me):
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