We might have a solution to Legacy's clue that's so crazy it might just be right.
Credit goes fully to Ghostie on the Discord, who used the # of strokes it took to write each number, and from that they got 17-19-20-19-9-4.
The ISBN for this is Celtic Fairytales and Legends.
Banshees are one of the main creatures in celtic folklore.
EDIT: Annnnd Legacy just debunked it. Welp.