Here is a Christy Mathewson signature on the back of a 1904 dinner menu for the 25th anniversary of The Union. Was auctioned off in the Grey Flannel June 2002 auction.
I did a Google search on 1904 Union Frontenac NY and came up with a book.google.com link to "Hayden's Annual Cyclopedia of Insurance in the United States 1904-1905" which mentions an annual meeting held at Frontenac NY, Sept 22, 1904 for the Union Insurance Company.
The same book also lists officers of the Union Insurance Company, and its different divisions. Some of whose signatures I was able to identify on the menu; such as J.F. Downing, J.W. Going, A.J. Harding, and Fred S. James.
More searching turned up several links mentioning Mathewson selling insurance in the off season during the 1900s. Including:
-A newsletter from his fraternity Phi Gamma Delta in which he is mention as a baseball player and insurance agent.
-Another link to a book that states in 1908 he joined the James Perry Agency of the Prudential Insurance Company.
-A third link to August "Garry" Herrmann's biography which has a passage in it that states Mathewson was an insurance broker in New York and in the winter of 1905-1906 Mathewson sold Herrmann a policy.
-A final link from Baseball; an Informal History states that "Within a few years of joining the Giants he was in the insurance business with an office at 20 Vesey Street in Manhattan"
So it's very likely that Mathewson would have been invited to this dinner.
Finally, the Giants were in town that day, they had a doubleheader with the Reds. Mathewson did not pitch that day and didn't pitch until the 24th.
So after researching the following can be said; (1) the dinner for sure happened, (2) signatures of some officers of the company can be found on the menu, and (3) Mathewson was know to have sold insurance in the years around the time of the dinner. Everything points to the high likelihood that Mathewson did attend this dinner and that the autograph is genuine.
I would like to hear what others think, am I way off here? I know it does not match up with the "classic" style of his signature, but there are just too many things adding up.