a couple thoughts:

Twain did own a very fine 1835 Martin guitar (purchased used around 1861),
and did play it, and owned it till his death in 1910; it was recently
valued at $15 million ! ! !  I cannot recall any eyewitness testimony of
him doing so in public (I may be wrong about that, though).  Still, there
is this little article available at Acoustic Guitar magazine claiming the
following:

He sang and played for newspaper men of the Nevada Territories, miners from
California's Gold Rush days and for passengers aboard Ajax, a clipper ship
bound for the Hawaiian Islands. But mostly he played for the willing women
of the West. In December 1866, Mark Twain brought his Martin 2 ½-17 guitar
aboard the steamer America and sailed from San Francisco to New York City,
where he would seek his fortune.

On the other hand, there is a fair amount of firsthand testimony of him
singing accapella, or singing as he played piano, in public at parties and
such, often in his home.  Joe Twichell, for one, describes Twain's
evocative singing several times in his Journals; often it was Twain singing
old hymns or things of that nature.  He also famously sang to his dying
wife in Italy.

Below is a photo of Mark sporting his vintage Fender, which he purchased
after listening to early Eric Clapton and Jeff Beck recordings:



-- 
Prof. Harold K. Bush
Professor of English
3800 Lindell
Saint Louis University
St. Louis, MO  63108
314-977-3616 (w); 314-771-6795 (h)
<www.slu.edu/x23809.xml>