What is believed to be the only surviving authenticated portrait of Billy the Kid went up for auction in Denver on Saturday and sold for $2.3 million.

The tintype on Saturday evening went to private collector William Koch at Brian Lebel's 22nd Annual Old West Show & Auction, where auction spokeswoman Melissa McCracken said the image of the 1800s outlaw was the most expensive piece ever sold at the event.

A 15 percent fee was added to the bidding price, making the selling price more than $2.6 million. Organizers had expected it could fetch between $300,000 and $400,000.

The tintype is believed to have been taken in 1879 or 1880 in Fort Sumner, N.M. It shows the outlaw dressed in a rumpled hat and layers of clothes, including a bulky sweater. He's standing with one hand resting on a Winchester carbine on his right side and a Colt revolver holstered on his left side.

Tintypes were an early form of photography that used metal plates. They are reverse images, and the Billy the Kid tintype led to the mistaken belief that Billy the Kid was a lefty. The myth inspired the 1958 movie "The Left Handed Gun", starring Paul Newman as Billy....