I understand what type of cipher it is, and the end step. As well as the crib that Kate uses for Charles which is “My Dear Charles”. Other than that idk how to either adjust the Polybius square, or the cipher text to get this beginning sequence and the rest be legible.
I’ve tried transposition (including mirroring certain sections of it) but I fear the true layer before the Polybius is the most difficult section.
So now I’m a bit stumped…