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.)