Historic cipher mapping

[MichelleL11]

And prior to 1549, if we're trying to get back the first half of the 1400s, doesn't that potentially lead to Byzantine sources?

First, I can confirm that the chances that Leonardo Bruni saw the Chapter X fragment are probably zero. His work that relied upon Polybius, Commentarii deprive bello punio used Book 1, chapter 7 through Book 2, chapter 34 (so a part of the "known" first five books of the time).  This analysis is in an academic article by G. Ianziti, You are not allowed to view links. Register or Login to view. Renaissance Quarterly, 51(2): 367-391, 377-378 (1998).  Bruni died in 1444, so a solid 100 years before the likely discovery date of the fragments that include Book X (see discussion below).

With that established, I took a quick look at what I could find beyond what pfeaster discussed and I found a bit of possible information from a source dated 1853.  Here's the clip from Charles Anthon, You are not allowed to view links. Register or Login to view. (p. 395):

   

It would seem that the fragment of Book X we're interested was from an unknown source from an unknown location at an unknown time that was found (or at least traversed through) Corfu.  This now Greek island was a colony of the Republic of Venice at the time (e.g. 1549).

This discussion seems to be at least partial basis for a later discussion of the various publications of Polybius in the preface of Evelyn Shuckburgh's 1889 translation into English of the text of Frederick Hultsch, which expands the possible mystery source into even more speculation with this:

   

You are not allowed to view links. Register or Login to view. is the link to the full preface.  You are not allowed to view links. Register or Login to view. is a link to an article partially about Casaubon, the translator who speculated it was Marcus Brutus' copy of Polybius (this paper also talks about Bruni so we've come full circle here).  This identification by Casaubon feels a bit too convenient to me to be probable, like the people who are reincarnated that are always Cleopatra or someone else equally famous in their prior lives, but I guess it's possible.

I haven't been able to find any contemporary attempts to find out more about this mysterious source of the fragments including Chapter X -- but it could be out there and does seem to be an interesting topic for study.  If I find anything else, I'll come back.