If you look at the fourth email dated January 10 2009, "Satoshi" appears to be quoting Hal Finney's overview, perhaps even cutting and pasting it into his own email to Adam. My point is, Hal Finney, as an American, probably did not write in British English, so the 'z' in 'decentralization' may not point to an error on the part of "Satoshi". In fact, I would think it weird if Hal Finney's quote by "Satoshi" here used the British spelling with an 's'.