As far as I know, the M has always stood for Matto since it only appears on the Italian soprafino style engraved Marseille decks.
However Mark Filipas surmised that each element of a Major image has a Hebrew alphabetic allusion.
For the Fool:
Folly — ThHLH, ThPLH
Fool — ThVRMVN, ThRDA
Wandering, Waywardness — ThYVBA, ThOH
Knapsack, Travelling bag — ThRMYL
To carry a knapsack — ThRML
Staff, Crutch — ThGDA, ThGRA
Feather in a cap — ThHLH
Lace (around his shoulders) — ThChRYM
Leaves of a fig tree — ThANH
Stitch, Seam — ThK, ThKYPH, ThPYRH, ThPR
Mark, Sign (on the back of shirt?) — ThYV (tav, yud, vav)
Staves of bone formation — ThOOMTh
Bare foot — ThChThYTh
Heel — ThPVCh OQB
Goat — ThYSh