Sources: Bitcoin paper, mail communication, mailing list posts (excluding the 2 posts in 2015), forum posts, code comments, website texts
Scores indicates that Satoshi's text are easy to understand/read (lower = easier, except for Flesch where higher = easier).
Missing archives: [cypherpunks-moderated], [openssl-users] (<2014), [cryptopp-users], [tor-talks], [e-gold-list], [p2p-hackers] [gsc] (gold-silver-crypto)
Weeks before the Bitcoin launch, Satoshi Nakamoto was in contact with famous cypherpunks and people on the cryptography mailing list. Most probably he lurked and/or was an active user on these cryptography/cypherpunks lists. Also, Reddit (/r/Bitcoin) had a lot of in-depth Bitcoin discussions between cypherpunks in the early days.
Assume Satoshi was already active on the internet (...under his real name), you could find the needle in the haystack in these data sources.