The gloves of Sonny Liston, front, and Muhammad Ali, rear, sit on a table, Jan 22. Ali autographed both his gloves and those of Liston several years ago. [Photo/IC]

How many of Jackson's signatures are in existence today isn't clear, but most experts agree the number is probably less than 100.

Autograph experts from two different authentication companies closely examined the signature on the photo of Jackson and determined it was the real thing, said Heritage Auctions said.

A baseball bearing Jackson's shaky signature brought $78,000 in 2011. Even a scrap of paper with his autograph sold for $23,100 nearly 25 years ago.

The photo of Jackson that sold Saturday was unknown in the sports memorabilia world until last year.