See this offering in this month's auction. It seems to say that that the letter is "unsigned" but then attributes it to Larry McPhail--possibly because there is a reference to Yanks not planning to sign any "Negroes" and he was club president. Still: I wonder why no certain of that. McPhail was indeed on committee at time studying if and when to break color barrier.
http://www.lelands.com/Auction/Aucti...Larry-MacPhail