I do think there is a link between the letter and late 19th century Theosophical thought. Mead was the foremost scholar of his day to advance Theosophy, something he embraced 1000%.
Whether the Theosophist movement and such like mystical orders (Masons, Rosicrucians, practitioners of Magick, popular Hindu celebrities and Spiritualists) were or are "right" or not is not the point. Knowledge of their distinct vocabulary might help, I think, unlock the mystery of the origins of the Letter of Theodore and its Secret Mark.
(Blatavsky, H P) The Theosophical Glossary (posthumously edited for publication by G R S Mead, 1918)
The Theosophical Glossary purposes to give information on the principal Sanskrit, Pahlavi, Tibetan, Pali, Chaldean, Persian, Scandinavian, Hebrew, Greek, Latin, Kabalistic and Gnostic words, and Occult terms generally used in Theosophical literature, and principally to be found in Isis Unveiled, Esoteric Buddhism, The Secret Doctrine, The Key to Theosophy, etc.; and in the monthly magazines, The Theosophist, Lucifer and The Path, etc., and other publications of the Theosophical Society.
It can be downloaded from Archive.org.
Edit: 11/29/2019 "bb" for "gg" in the word before "-de-gook."