http://www.nitrosyncretic.com/nsp_rah-telepathy.php http://www.nitrosyncretic.com/nsp_rah-telepathy.php says the "Mental Telegraphy" is from Expanded Universe,

http://www.nitrosyncretic.com/nsp_rah-telepathy.php http://www.nitrosyncretic.com/nsp_rah-telepathy.php says the "Mental Telegraphy" is from Expanded Universe, and includes a copy of Twain's text. I haven't reread Heinlein for decades and don't remember the reference. An archive.org search of the book wasn't fruitful.