Noticed this thread after I googled William S. Marhall.
Only a bit of information, but I found a letter dated 1884 either from Salt Lake City, UT or to a woman in Salt Lake City. I couldn't read all of the handwriting, but it was signed distinctly at the bottom: William S. Marshall. Since it was dated 1884, I noticed that this would be past the estimated death date in the posting. Hope that helps.
By the way, the letter was in a book store in Washington, D.C. They sell a lot of old historic books, lithographs, and prints. Occasionally, as was the case here, they have old signed letters.