At a flea market six years ago, a North Carolina lawyer named Frank Abrams unknowingly bought a rare photograph that experts say shows Billy the Kid relaxing with the man who would eventually kill him.

Billy the Kid, one of the best-known outlaws in American history, is thought to be there in the back, second from the left. The man all the way to the right is Pat Garrett, who would soon become the sheriff of Lincoln County, N.M., and — the story goes — shoot the outlaw dead in a darkened room.

The men appear on a tintype that is about the size of a man’s palm. (Tintypes are photographic images produced on thin sheets of metal. They became popular during the late 19th century and, as in this case, often show a version of reality that is reversed, left-to-right.)

In 2011, Mr. Abrams saw nothing more than a group of five men who looked like cowboys. He considered it a strange find, since most tintypes that ended up in North Carolina harkened back to the American South, not the Wild West. So he bought it for $10, he said in a phone interview, and put it up in his home. It hung in a room where he hosted Airbnb guests. Mr. Abrams used to jokingly tell them that it was a picture of Jesse James.