Let me add something to this old thread:
Here we have an example with a "modern" calotte dated 1652 and marked a with an "M". As this piece is from the arsenal of Kurfürst Friedrich Wilhelm von Brandenburg, it is one more hint that the marks "L", "M" etc. is not an arsenal but a makers mark.
https://www.hermann-historica.de/de/.../lot/id/138656