The rarely found nickname

Rarely do I come across the nickname of a soldier in the course of my research. When I do, I'm ecstatic, because it makes my profiles more personal. When I do not, I'm disappointed. Case in point: Thomas R. Clark, a Vermonter who became a signal officer in the Union army, is my current subject. The Gettysburg National Military Park owns a collection of his personal items. Included in the collection is an account book, or business ledger, that belonged to his brother, William J. Clark. In the book he signs his name as "Willie." A nickname! But Thomas, my subject, always signs his full name. More than likely he was called "Tom," but I've no way to prove it, and so cannot include it in my profile.