The correspondence mentioned is available online.
http://ucispace.lib.uci.edu/handle/10575/1158%c2%a0


They seem to be talking more about Jaynes work in probability theory and statistical mechanics. Didn't see relevant comments on quantum theory, but mainly scanned the docs. 

Given Jayne's interest in the foundations of quantum theory, it seems extremely unlikely to me that he was unaware of MWI. I've read most of his papers since around 1980, and can't recall a mention anywhere. Surely he was aware, and surely he had an opinion. 

Buy Buy - Dan Davis

>________________________________
> From: Gwern Branwen <gwern0@...>
>To: etjaynesstudy@yahoogroups.com
>Sent: Sunday, April 29, 2012 4:45 PM
>Subject: Re: [etjaynesstudy] Jaynes on MWI?
>
>On Sun, Apr 29, 2012 at 7:23 PM, Daniel <buybuydandavis@...> wrote:
>> Does anyone have any references for Jayne's opinion on the Many Worlds Interpretation of quantum theory?
>
>If anyone has access to _The Everett Interpretation of Quantum
>Mechanics: Collected Works 1955-1980_, ch18 would be worth checking:
>http://lesswrong.com/lw/c3g/seq_rerun_quantum_nonrealism/6h7e
>
>--
>gwern
>http://www.gwern.net
>
>
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