How likely is it, that he had a deeper knowledge of cryptography in the 1860ies? But of course, if he had, it's still likely, that he used something more sophisticated.
http://en.wikipedia.org/wiki/History_of_cryptography#Cryptog...
Given the fact, that "12345" is still a common password in 2012, I would at least give it a shot. ;)
The reason I suggest a homophonic cipher (or similar) rather than something like the much more secure Vignere in use for telegraph messages (only recently broken for the time) is because the Vignere system is more complicated, and requires more working out than I would expect someone writing a private message to tolerate.
Homophonic systems, on the other hand, are fairly easy to invent and remember on a personal basis, and can offer some security against amateur analysis. Though the technique for solving them was known, they could still prove robust - the partially homophonic cipher of Lous XIV was still unbroken at this point, despite being over a century old.
I'm very much an amateur (though I have read that book) but it doesn't seem that ridiculous to suggest a homophonic cipher. At any rate, It's only something to consider if it turns out to be something more complex than a simple substitution cipher.