Here's another place where O.Henry uses "coma mott", in a way that sounds as though it might mean 'cemetery':
""Thirty-two years I have lived on the Rancho Cibolo," said Tia Juana. "I thought to be buried under the coma mott beyond the garden before these things should be known. Close the door, Don Ransom, and I will speak. I see in your face that you know."