Garry Herrmann, president of baseball's National Commission at the time, reported that Creamer said he was representing 3 Giants. In the official minutes they attepted to redact the names by blacking them out with ink, but the names were still clearly visible - "McGraw, Bresnahan, and Matty".
I read about it in a book called 1908. But the evidence is the "Minutes of the Annual Meeting of the National League, February 1909" - page 169 is the page with blacked out names still visible. Currently on display at the National Hall of Fame Library.