A photograph of the outlaw Billy the Kid bought for $2 at a Fresno, California, junk shop five years ago is apparently the “Holy Grail of Western Americana” and worth $5 million, experts say.

The 4-by-5-inch tintype shows Henry McCarty, better known as William H. Bonney or Billy the Kid, and several members of his gang, the Regulators, as they played croquet with friends, family and significant others in the summer of 1878, according to Kagin’s Inc., a numismatic firm that specializes in U.S. gold coins and collectibles in Tiburon, Calif.

The photograph is believed to have been taken a month before the Lincoln County War in New Mexico Territory. The rare find has been appraised and insured for $5 million.

When experts initially inspected the photo, they had their doubts about it.

“When we first saw the photograph, we were understandably skeptical – an original Billy the Kid photo is the Holy Grail of Western Americana,” Kagin’s senior numismatist David McCarthy said in a statement. “We had to be certain that we could answer and verify where, when, how and why this photograph was taken.”

The firm assembled a team of experts to examine every detail of the photograph to ensure its authenticity because “simple resemblance is not enough in a case like this,” he said.

A methodical study of the photograph went on for more than a year. McCarthy said evidence of the image’s authenticity is overwhelming.

The unique photograph is the only other authenticated portrait known to exist of the notorious Wild West figure. The other photograph is a 2-by-3-inch tintype taken at Fort Sumner in New Mexico Territory in 1880. The image sold for $2.3 million in 2010.

“The historical importance of a photograph of Billy the Kid alongside known members of his gang and prominent Lincoln County citizens is incalculable – this is perhaps the single most compelling piece of Western Americana that we have ever seen,” Donald Kagin, president of Kagin’s, said in a statement. He is representing the sellers, Randy and Linda Guijarro.

Kevin Costner is narrating a documentary set to air Sunday on the National Geographic Channel about the journey to authenticate the photograph and the history behind it.