RE: the Martin supposed to have been Clemens'
The assumption that Clemens bought a guitar from Martin and transported
it to Nevada seems to ignore any number of possibilities, including
winning a guitar in a card game or buying it in a pawn shop after
arriving in the West.
Yes Martins are pricey, but I bought mine used for a fraction of the
list price in 1974, a time when guitars were much more in demand than
they no doubt were in the middle of the 19th C.
While proof of provenance is the only way to verify the claim that the
instrument in question was Twain's, the doubts raised aren't strong
refutations of the possibility.