It never ceases to amaze me, when asking someone for a signature, how many people ask for someone's "John Henry."
I could go on for a while citing sources, but it's pretty simple:
It was John Hancock with the flowery signature on the Declaration of Independence. John Henry, whose life is preserved mainly in folk lore was a railroad worker. By the most popular accounts he pit himself against a steam-driven hammer machine in a bid to save the jobs of his fellow rail workers.
So unless you want them to hit something with a sledgehammer, ask for their "John Hancock"