Sharpies didn't exist when those pistons were stamped, most likely. If I was going to do something like that, I'd do it before I removed them from the engine. No idea why there's an "M", but the piston/rod assembly has to go back onto the correct connecting rod journal to keep the bearing fit correct, so numbering them is a really good idea..
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!