I could not find meaning of these words.Can you?
"Santa's father," explained Webb gently, "got her to promise that she wouldn't write to me or send me any word. That heart-and-cross sign was her scheme. Whenever she wanted to see me in particular she managed to put that mark on somethin' at the ranch that she knew I'd see. And I never laid eyes on it but what I burnt the wind for the ranch the same night. I used to see her in that coma mott back of the little horse-corral."

Edit: I found the same word in another O.Henry story, so it's not a misprint. Also seemed to me a small place or area. A Spanish dictionary gave Spanish "coma" = English "coma" , no defintion of mott.

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."

O.Henry again uses the word "mott" for which I can find no dictionary definition. Lots of use as family name.

"They were rolling southward on the International. The timber was huddling into little, dense green motts are distances before the inundation of the downright, vert prairies. This was the land of the ranches; the domain of the kings of the kine."

Here's another:
"The road perished, and the buckboard swam the uncharted billows of the grass itself, steered by the practised hand of Raidler, to whom each tiny distant mott of trees was a signboard, each convolution of the low hills a voucher of course and distance. "

Still another instance, this time "comma mott".
""You came up the trail from the Double-Elm Fork," he said promisingly. "As you crossed it you must have seen an old deserted jacal to your left under a comma mott."

This time it may mean a clump of trees of some kind.