UPDATE: Reader Chuck Lyons offers up a wonderful contribution to this story. Forty years ago he visited the Library of Congress and got a copy of a Houdini letter on this very letterhead. What's amazing is the letter itself is about the letterhead! Houdini writes:
My 1909-10-11 Letter head. I never liked this one. I never had to write for work, but wanted a note head for general business. Harry Houdini, May 22/1916
Fantastic! Thank you Chuck.