Where does the Theban sign for J come from, and does Trithemius's ascription of the script to Honorius of Thebes rest on anything but his own word?
The three attributed-author facts recorded on Theban J are all marked unknown, and this is the question they leave open. Trithemius published the script in Polygraphiae libri sex in 1518, crediting Honorius of Thebes on the authority of Pietro d'Abano, but the credit has never been corroborated. The ascription is not known to appear in any writing attributed to d'Abano, the script is not known to appear in any manuscript of Honorius except British Library Sloane 3853, which names Agrippa as its own source, and no publication before 1518 has been found to carry it. A Trithemius era invention is widely assumed but has never been demonstrated, and no rival candidate has ever been named, which is why this is unknown rather than debated. It is one question about the whole Theban script rather than about this letter alone, and it is recorded on each of the twenty six letters because the atlas stores the attribution separately for every letter.
What would resolve this A codicological survey of pre-1518 manuscripts in the ars notoria and Solomonic grimoire traditions for any script matching the Theban letterforms, together with archival work on Trithemius's own circle, including his students and correspondents such as Agrippa, and a collation of the Polygraphiae chart against the earliest surviving impressions. One dated witness earlier than 1518 would settle the priority; a draft, a letter or a working paper would settle the authorship.
Palaeography and codicology of late medieval Latin manuscripts, history of the early modern printed book, history of Renaissance occultismWikipedia contributors, Theban alphabet (reference article)
Who first printed a Theban sign for J distinct from the sign for I, and where did that form come from?
J was not a separate letter of the Latin alphabet Trithemius was ciphering in 1518, and the earliest Theban sets carry no sign for it: the sign for I served for both. The Theban J in circulation today is a later supplement, usually described as adapted from the I, but nobody has identified the chart or edition in which it first appears, or the hand that made it. The modern twenty six letter Theban set therefore contains one letter with no traceable origin at all.
What would resolve this A bibliographic collation of printed Theban charts in sequence, from Polygraphiae in 1518 and Agrippa's De occulta philosophia in 1533, through the eighteenth and nineteenth century grimoire reprints, to the twentieth century witchcraft revival, to locate the earliest surviving witness of the supplementary sign and to name the edition that introduced it.
History of the printed book, analytical bibliography of occult literatureWikipedia contributors, Theban alphabet (reference article)