Went through some of my correct era reels/reel feet - 5 reels, 1 > face, 4 > back; 2 reels were new and I know they were never taken apart, they all face to the back. No reels have M's showing on the guides, several were changed to LHW by me, and I would have put them back the way they initially were installed.
Certainly not proof, just anecdotal.

Thanks guys! Anyone have a guess what the "M" stands for?

" Anyone have a guess what the "M" stands for?"

My best guess is the obvious, M for Medalist, but it's a part designation like the B on the reel foot, which might stand for bottom, but could also help employees sort required parts in a room full miscellaneous parts for several different reels, it's possible that each mechanic had a parts diagram with the letters showing where the parts fit. It could be designating one of a series of design changes for that part, M=13 changes, B=2 changes etc. But because it's hidden on the assembled reel, we know it was meant for factory use, with no meaning to the consumer.