Mar. March.

marg. Margin, Marginal.

Marg., Mrgt. Margaret.

Marq. Marquis.

mas., masc. Masculine.

Mass. Massachusetts.

Math. Mathematics.

Matt. Matthew.

M.B. Mark of the Beast, as in 'M.B. waistcoat.'

M.B., Medicinæ Baccalaureus=Bachelor of Medicine.