I always try to learn easy ways to remember things. The word "stamp" has an "M" in it. Date coding that is Stamped uses the "M" for December because it skips the "I" (since it can be confused with a #1). Date coding that is CAST uses the "I", so December is "L".
I've never seen a cast "M", but if one exists, I'd sure like to see one.
Verne