Martin J. Ball, John H. Esling, and B. Craig Dickson. "Revisions to the VoQS system for the transcription of voice quality." Journal of the International Phonetic Association 48.2 (August 2018): 165-171.
M / The oldest form of this letter consists of five parts, ꟿ. This is found on the Fibula Praenestina, also in the Duenos Inscription. It gives rise to [forms of M], which in the days of the Republic stands as the abbreviation for Manius, as distinguished from Marcus with abbreviation [form of M]. The prevailing form under the Republic, and the standard letter of the best period is that made with oblique lines reaching to the base of the letter.