How much survives and how much is lost to history?

[Mark Knowles]

It's a tough journey from there-and-then to here-and-now...not just victims of the predictable dangers of fire, flood, war, rodents, etc.

The first item on your list rings bells. The 1447 Milan fire in the Castello di Porta Giovia and the 1483 Venice fire in the Palazzo Ducale are responsible I think for the destruction/disappearance of a lot of cryptographic material from the early 15th century. Those two fires have really hampered my research as both Milan and Venice were two of the major users of ciphers in the early 15th century. So one is forced to look elsewhere for evidence of the cryptographic techniques used by these two states.