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[Daniele]

I would suggest contacting the You are not allowed to view links. Register or Login to view., which holds the archives of the chancellery of the Visconti Duchy (1395–1447) and the subsequent Sforza Duchy (1450–1499). The Sforza chancellery made extensive use of ciphers for correspondence (see source below), and I’ve personally seen some of these letters at the Archive. It is possible that the Archive holds similar enciphered letters by the previous Visconti chancellery.

The relevant archival fonds for the time period you are interested in should be You are not allowed to view links. Register or Login to view..

The You are not allowed to view links. Register or Login to view. contains many digitized letters, but alas only from 1448 onwards.

You may contact the State Archive via email or phone:

• Email: You are not allowed to view links. Register or Login to view.
  
• Phone: You are not allowed to view links. Register or Login to view.
  

Relevant source (unfortunately I don’t have access to it at the moment):

• Cerioni, Lydia (1970). La diplomazia sforzesca nella seconda metà del Quattrocento e i suoi cifrari segreti. Il Centro di Ricerca Ed., Rome.