It's not every day that a $2 thrift shop purchase yields a historical find worth $5 million, but then again, it's not every day that you come across a rare photograph of one of America's most notorious outlaws.

The 4x5 tintype, which was found by within a miscellaneous lot bought by Randy Guijarro from a Fresno junk shop in 2010, shows Billy the Kid (born Henry McCarty on Sept. 17, 1859) playing a game of croquet with members of his gang, the Lincoln County Regulators, during a late summer wedding in 1878. If you zoom in on the image, Billy the Kid is seen standing on the left and leaning casually against a croquet mallet.

The image's compelling origins were uncovered by Kagin's, Inc., a numismatics firm that specializes in evaluating and appraising rare coins, currency and other historical items.

"When we first saw the photograph, we were understandably skeptical – an original Billy the Kid photo is the Holy Grail of Western Americana," said David McCarthy, a senior numismatist at Kagin's, Inc. "We had to be certain that we could answer and verify where, when, how and why this photograph was taken. Simple resemblance is not enough in a case like this – a team of experts had to be assembled to address each and every detail in the photo to insure that nothing was out of place."

In the end, it took more than a year of investigation to definitively confirm the image's authenticity, and it has since been appraised and insured for a staggering $5 million. To put that into perspective, the only other confirmed photograph of Billy the Kid — a 2x3 ferrotype captured at Fort Sumner, New Mexico in 1880 (seen at left) — was sold in 2010 for $2.3 million.

"The historical importance of a photograph of Billy the Kid alongside known members of his gang and prominent Lincoln County citizens is incalculable," the firm's president, Dr. Donald Kagin, said. "This is perhaps the single most compelling piece of Western Americana that we have ever seen." 

The fascinating find will be the subject of a two-hour documentary airing Oct. 18 on the National Geographic channel. Narrated by Kevin Costner, the documentary will recount the story behind the image's initial discovery and eventual authentication