French archives BnF fr. 3979 (Gallica), fol.92, has an undeciphered letter from the Duke of Mantua to the Duke of Nevers dated 17 September 1590. Somehow, the same cipher is used in the following letters in BnF fr.3983 (Gallica).
(fol.40) Carlo Barberini (Charles Barberini) (Wikipedia) to Filippo Sega (Philippe Sega) (Wikipedia), Bishop of Piacenza, 1593
(fol.43) An unsigned note, 1593
(fol.83) Duke of Mantua to Duke of Nevers, 2 January 1593
The interlined decipherment of these allows reconstruction of the cipher, which in turn allows reading the undeciphered letter from 1590.
The reconstructed cipher is as follows.