oh dear.
I didn't really expect this one to be difficult, but I did find the idea amusing.
He has these characters who are lacking parts of their anatomy (One-hand, nine-fingers etc) and then he implies that he is, in at least some sense, similarly disabled. But only in some sense. He doesn't describe how he left his left nostril behind after a brawl on platform 9 3/4 of Oxford station or anything.
Furthermore he describes this in one of the main books, (and not the letters). Remember he does not do cameo performances in his own stories, so it must be outside of the main text.
If I give you any more clues it'll be the page and line numbers.
(I do hope that this exists in all prints of the book, but I'm pretty sure it does.)