Mark Twain's Nom de guerre - First Use

Samuel Clemens' first use of the nom de guerre "Mark Twain" (?) It seems he assumed the name from Mississippi River pilot Captain Isaiah Sellers:

"I was a cub pilot on the Mississippi River then, and one day I wrote a rude and crude satire which was leveled at Captain Isaiah Sellers, the oldest steamboat pilot on the Mississippi River, and the most respected, esteemed, and revered. For many years he had occasionally written brief paragraphs concerning the river and the changes which it had undergone under his observation during fifty years, and had signed these paragraphs “Mark Twain” and published them in the St. Louis and New Orleans journals. In my satire I made rude game of his reminiscences. It was a shabby poor performance, but I didn’t know it, and the pilots didn’t know it. The pilots thought it was brilliant. They were jealous of Sellers, because when the gray-heads among them pleased their vanity by detailing in the hearing of the younger craftsmen marvels which they had seen in the long ago on the river, Sellers was always likely to step in at the psychological moment and snuff them out with wonders of his own which made their small marvels look pale and sick. However, I have told all about this in “Old Times on the Mississippi.”

"The pilots handed my extravagant satire to a river reporter, and it was published in the New Orleans True Delta. That poor old Captain Sellers was deeply wounded. He had never been held up to ridicule before; he was sensitive, and he never got over the hurt which I had wantonly and stupidly inflicted upon his dignity. I was proud of my performance for a while, and considered it quite wonderful, but I have changed my opinion of it long ago. Sellers never published another paragraph nor ever used his nom de guerre again."

Via: "10 September 1906: Paragraph 5," in Autobiography of Mark Twain, Volume 2. 2013, 2008. <http://www.marktwainproject.org/xtf/view?docId=works/MTDP10363.xml;style=work;brand=mtp;chunk.id=dv0050#pa001690>