(Reuters) - Karl Kissner and his cousin were rummaging through their late grandfather's attic in Defiance, Ohio, and at first ignored a box of baseball cards depicting long-ago stars Ty Cobb, Cy Young and Honus Wagner. Turned out, their discovery is worth millions.

The 51-year-old restaurant owner compared his find in February to an art collector locating a long-lost "Mona Lisa" by Leonardo da Vinci.

"We're considering it the most significant find in the history of the hobby," said Chris Ivy of Heritage Auctions in Dallas, which is handling the sale of the cards.

Kissner, his cousin and 18 other members of their extended family are set to split some $2.8 million from the cards' sale, though their exact amount will be determined at auction.

The 700 pristine cards discovered will double the number of 1910 cards known to be in existence, Ivy said. Most cards from the era were roughly handled by children and were often stained by the caramel candy they were packaged with.

Many baseball card collectors do not bother with the 1910 edition because examples are so scarce, Ivy said.

The most famous baseball card is the 1909 Honus Wagner card, of which there are 75 in existence, one of which sold for $2.8 million in 2007, Ivy said. Wagner asked the tobacco company producing the 1909 edition to stop making it, either because he did not want to promote tobacco or because he felt under compensated.

Wagner, nicknamed the Flying Dutchman, was a Hall of Fame shortstop and outfielder who played for the Louisville Colonels and the Pittsburgh Pirates.

The 1910 Wagner card may not be worth as much, but it is nearly as rare, with the 44 examples found in Defiance increasing the total number to 104 cards, Ivy said. He estimated each may bring in $200,000 or more.

Among the mysteries is who manufactured the cards, which were given away as promotional items by traveling salesmen.

Kissner, who runs a family-owned restaurant that began in 1928, said his grandfather was a butcher in Defiance who sold candy and gave away baseball cards as a marketing strategy. The cards were probably left over and forgotten inside a cardboard box normally used to hold women's clothing.

The grandfather, Carl Hench, died in 1944, and his wife passed away in 1976.

"My aunt, who never married, then lived in the house and she was a bit of a pack rat, so nothing ever left the house," Kissner said.

When the aunt died, Kissner invited family members over to engage in "a trip down memory lane" by going through their belongings that ended up uncovering treasure.

The most valuable 37 cards will be auctioned off August 2 at Baltimore's Camden Yards baseball field during the National Sports Collectors Convention. Bids are being taken online at www.HA.com, and Heritage Auctions will spread out the sale of the remaining cards so as not to flood the market.

(Reporting By Andrew Stern; Editing by Greg McCune and Cynthia Osterman)