Where does the Bohairic Coptic letter khai come from?

Why it is open. Six of the seven Egyptian additions to the Coptic alphabet are taken from Demotic and their sources are not in dispute. Khai is the exception. It appears in Bohairic and not in Sahidic, which has thirty-one letters where Bohairic has thirty-two, and accounts of where its shape came from differ. This atlas states the letter, its dialect distribution and its sound, and does not state a derivation.

What would resolve it. Forms collected from dated Old Coptic and early Bohairic manuscripts and compared against the Demotic sign repertoire, with the dialect distribution of the letter mapped alongside. A published palaeographic study of that comparison would settle it or would establish that the evidence does not.

Field. Coptology and Egyptian palaeography