While it's not easy to see what MT erased on the signature of this 28 December 1868 letter to Olivia Langdon, the Mark Twain Project has identified the remains of "Mark" under the name "Samuel." He never signed letters to Livy as "Mark," but "Samuel" is almost as rare -- letters to Livy are typically signed "Sam" or "Saml." There's no way to say for sure what his reasons were for either the name he first used or the one he substituted.
This page appears here with permission of the Mark Twain Papers, UC Berkeley. To see the entire letter, with annotations by the Project, use the link below right, and search for MS: CU-MARK, UCCL 02768.TRANSCRIPT OF THIS PAGE | MARK TWAIN PROJECT