Rudy started by making knives from Studebaker springs (and before then, he was a blacksmith for the Cav, and whatever was handy), pretty sure he gave saw blades a whirl, Studebaker spring stock dried up, and he went with 1095HC, not sure if the Sharon stuff, late 1950s/earliest 1960s, and the M marked the change to that last steel, as far as I know. No idea as to significance unless to mark a different temper from other harder saw steel at the time, ie Medium. It surely would have been tougher, less easy to break/chip, and why the main course ever since. Pappy is welcome to blow me out of the water, now...