Historic cipher mapping

[pfeaster]

First, I can confirm that the chances that Leonardo Bruni saw the Chapter X fragment are probably zero. 

That does seem to be how it's looking.  Here's a further You are not allowed to view links. Register or Login to view. in Arnaldo Momigliano's Essays in Ancient and Modern Historiography that leans strongly in this direction while acknowledging a (seemingly weak) argument to the contrary:

   

On the other hand, that point about Byzantine sources is a fair one: Polybius could have been read and known continuously in Constantinople up to 1453.

I see that there's a monograph by J. M. Moore about the manuscript tradition of Polybius which could be worth a look, but in the meantime, there's also a survey of manuscripts online You are not allowed to view links. Register or Login to view..  The name Excerpta Antiqua seems to have been given to the collection of excerpts including the pieces of Book X, which may help in searching for references to it.  Of manuscripts of the Excerpta Antiqua mentioned on that website, one copy listed as from the fifteenth century (at the BnF) is online You are not allowed to view links. Register or Login to view., but identified in the catalog record as sixteenth century.  Other possible early copies at the Vatican, but unclear from just this how early and when they got there.

Polybius certainly isn't the only known older source for a cipher with a reduced character set.  As JKP has pointed out, any numerically-based notation can have a similar effect.  And I don't mean to beat a dead horse by rehashing old points about the sizes of cipher character sets, much less to claim that there's anything original about the observations themselves.  But on the current topic of "mapping," I'd assume we'd want to be especially attentive to ciphers involving reduced character sets, however they achieve them.  So if and when anyone's going hunting, it's a good (if obvious) thing to know to watch for.