Combining with the mating-type peptide pheromone M-factor and transmitting the signal across the membrane to initiate a change in cell activity. M-factor is a nine-membered oligopeptide that consists of tyrosyl, threonyl, prolyl, lysyl, valyl, prolyl, tyrosyl, methionyl and methyl S-farnesylcysteinate residues joined in sequence, and is a peptide pheromone released by Schizosaccharomyces pombe cells of the cellular mating type Minus.