This illustration pre-dates the famous series of photos Houdini took with the can in June of 1908. Those photos are, incredibly, the only known images of Houdini with his Milk Can. I don't know exactly when Houdini created his famous Milk Can poster.
Below is an ad for Houdini's week in Pittsburgh referencing his "unprecedented and hitherto unknown and unnamed mystery."
UPDATE: Fred Pittella has shared a program from the Columbia Theatre in St. Louis during the first week Houdini presented his Milk Can and it shows he offered the $25 award for the best name during that engagement as well. By the way, it doesn't appear he ever actually found a name. While we call it "the Milk Can" today, Houdini's advertising continued to refer to it only as the "Galvanized Iron Can filled with water and secured by six padlocks."