Just a thought --

Justin Kaplan, author of the great biography Mr. Clemens and
Mark Twain, is I believe married to writer Anne Bernays.  She is
the grandniece of Sigmund Freud.

I do not know how to contact Mr. Kaplan, except perhaps through
his publisher.  I'm thinking that he lives in Cambridge, Massachusetts.
But it would not be a surprise to me if he came across whatever material
there is concerning Freud and Twain as he did the research for his book.

I would also imagine he would have more than a passing interest in
exploring it as well, if for no other reason than the family connection.
But that's only a supposition on my part.

Good luck in your search!

Roger Durrett