Thanks to drxx and quisling, I now agree that the reflective letter is not ambiguous but it is pretty obscure! I have read and listened to the source material many times but would never have identified it from memory. Hats off to anyone who got it without referring to the illustration. It could have been the 11th letter or the 10th, as some have suggested. Anyway, I am no longer stuck and in need of help,so many thanks.