A Polyalphabetic Cipher of Mary, Queen of Scots

I added a polyalphabetic cipher used by Mary, Queen of Scots, in "Ciphers of Mary, Queen of Scots". (I learned of this cipher from Oliver Storz.) It was used in September 1568, a few months after Mary fled to England. This is the only polyalphabetic cipher used by Mary known to me.
Another interesting thing in this cipher is that the letter remained unsolved until the twentieth century, when it was solved in the French foreign ministry.