Where does the Theban sign for D come from, and does Trithemius's ascription of the script to Honorius of Thebes rest on anything but his own word?

Why it is open. The three attributed-author facts recorded on Theban D 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 it. 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.

Field. Palaeography and codicology of late medieval Latin manuscripts, history of the early modern printed book, history of Renaissance occultism