They left out Billy's sweater!

In the famous photo of Billy the Kid, the one that was reversed and gave rise to the mistaken belief that Billy was left-handed, he's wearing a sweater. I've always found that interesting. I'm a frontier history buff, and in all the photos I've seen of cowboys, gunmen, et al from the late 19th Century, I don't think I've ever seen anyone but Billy wearing a sweater.