N or M?

Agatha Christie

The man circled slowly round the houser Tidy little place, he thought Quiet, peaceful Then a long-drawn grunt came to him.

Hallo, so they kept pigs did they? Funny— seemed as though it was underground. An odd place to keep pigs. No, it was some having a bit of shut-eyeBut who’d want to kip in a cellar?

He moved right up to a wall of the house. There, that’s where it was coming from–- through that grating Grunt, grunt, grunt. Snore, snore, snore… now that reminded him of something. “Cor!” said the man, “that’s what it is— S.0.S.” Kneeling down, he began to tap out a message on the iron grill of the little cellar window…