Howard wrote a letter (#346) to Jack Byrne on April 21, 1936 including a draft for a story. The story ends in the middle of the page and seems to be a part of the letter so it's probably never sent. Howard began writing this story but did not finish it; Glenn Lord assigned the unfinished, untitled draft the title “The Diablos Trail.”

Mr. Jack Byrne,
Editor Munsey Publications,
New York City.

Dear Mr. Byrne:
My agent, O.A. Kline, tells me that you have suggested that I try my hand at a series of humorous yarns for Argosy, on the general type of the Breckinridge Elkins stories.
I have in mind a new character, Pike Bearfield, of Wolf Mountain, Texas, about as big, dumb, and ludicrous as B. Elkins. Here is an outline of the first yarn of the series, as I have conceived it. It begins in the form of a letter written by Bearfield to his boss, William Westphal.

You can read the rest of the incomplete letter in The Collected Letters of Robert E. Howard Volume 3.