A Spanish Love Letter of 1596 among the Medici Cipher Keys (DECODE R7397)

A volume of Medici cipher keys in the Archivio di Stato di Firenze contains a letter of two pages written entirely in signs, with no address, no signature in clear and no decipherment (DECODE R7397). I transcribed its 1,300 signs and found by hand, from three occurrences of the group that turned out to be vuestra, that it is Spanish, enciphered by simple substitution: the digits 4, 3, 2 and 1 stand for e, i, o and u, a long s for a, and about twenty other signs for the consonants. A solver given only the ciphertext and a model of Spanish found the same key. The letter is dated July 1596 and ends "Vuestra hasta la muerte". The writer is pregnant and afraid of dying in childbirth. She writes to her absent lover about a suitor who has been pressing her with the help of a teatino, a marchioness and a viscountess, who has sent her a gold jewel, and whom she now sees at the window where she used to see her lover. Underlines and marks in the margin look like an earlier attempt to break the cipher. I discuss whether the recipient was Don Pietro de' Medici, whom Philip II sent from Madrid to Rome in 1596. Files: the paper (PDF). No images of the manuscript are included; the letter is held by the Archivio di Stato di Firenze.