Secret page

You are on the SECRET PAGE !!!
Well, it's a little bit empty, it's useless... but you found it, and that is great!
Come on, keep the rythm!

In order to find the next mystery letter, you have to wear out the grey matter. I started this sudoku trying to find it but it's too dificult for me. Let's try to complete it!

sudoku
Used letters: ABJMORSWX

Next hint: "Sometimes, when you are lost, it's good to step back..."