[She frowns at it, then quickly flips to a clean page and writes down the entire alphabet in the cypher, in order, and compares the mark.]

It seems like the letters that contain some part of this stroke are A, M, N, R, V, W, and X. Of those, I'd consider V, W, or X to be most likely. Any of the others would need to be written in some way other than the standard left to right.