The original spirit message from Houdini's mother delivered via "automatic writing" by Lady Doyle during the famous Atlantic City seance has been unearthed in David Copperfield's International Museum and Library of the Conjuring Arts. The 17-page message survives complete, along with a cover page written by Houdini. While never lost, the message has remained largely unseen, and its existence was not generally known.
As detailed in this post, the Doyles believed Houdini had been wholly convinced by the message. But Houdini was troubled by the lack of specifics and that the pages were written in English. He and his mother only communicated in German. He eventually told the Doyles that he did not believe the message had come from his mother.
Nevertheless, Houdini appears to have kept the message for the rest of his life. The first two pages were reproduced in the 1932 book Houdini and Conan Doyle: The Story of Strange Friendship by Bernard M.L. Ernst and Hereward Carrington. But after that, the original message seemed to vanish and only transcriptions survived.
I now think it's possible these pages passed from Bernard Ernst to magician and collector John Mulholland. David Copperfield acquired the Mulholland Collection in 1991, which seeded the magnificent museum he has today. So Mama has been home the entire time!
Thanks to Bruce Averbook for making this find and David Copperfield for allowing me to share. You can watch video of the discovery on my Patreon.