Nobody knows for certain who authored Sefer Kol Bo (ספר כל-בו), and various theories exist as to who, and when, it might have been. (For a brief example of such theories, see here). Likely it was in the 14th century, although it may well have been slightly earlier.
Frequently throughout the text, the author refers to the opinions of a certain ר"מ. This acronym could stand for Reish Mesivta ("the head of the academy"), or it could be a person's name (R' Meir, R' Mordekhai, R' Menachem, etc). Does anybody know to whom the author of Sefer Kol Bo might have been referring?
It's important because, in quoting Sefer Kol Bo, later compilers have often attributed the opinions within it to this ר"מ - such as the Bet Yosef, for example, in Tur OC 91:3.