A few years ago a friend of mine showed me a guitar magazine that included
a one-page note about a Martin guitar apparently owned by Mark Twain. I
posted the text of the article here, and some doubted the authenticity of
the guitar in question; I think it was
finally confirmed, but I don't recall exactly.
  Now that same friend has discovered a site that includes another short
note about the guitar, including some documentation that I don't think the
earlier article had. And this one also has a photo, a reproduction of the
page I saw in the magazine. So I thought I'd
pass it on. This is the site:

http://www.themomi.org/museum/articles/marktwain/1.html

Bob G.