As for the WHY of the mark, my guess has to do with breakage. Ruana was always known for, and even a motto of sorts, making a knife which would not let a guy down in the middle of nowhere, unlike most all of other designs, as there simply was nothing to shrink, rot, crack, or come loose to the design. Which led to some mighty harsh use among buyers.

I kinda guess the M stamp was in answer to such abuse, where, about inconceivable the M stamped stuff could bust under any sane use. Where L6 saw steel might hold a great edge but might chip on bone. Just a guess, mind you, but L6 blades might have been far easier to get replaced than the M stamped blades, where Rudy might have rightly said you were breaking an M stamp blade nearly on purpose.

All conjecture, unproven, unverified, which means it now will be repeated as fact over the entire internet for the next 20yrs, even showing up in a Ford antique tractor blog.