Polo Grounds memorabilia, game-used balls up for auction

This bag of balls can be yours — if the auction price is right!

Back in the day, fans at the old Polo Grounds hurled horsehides onto the diamond — but not because a hated opposing player had launched a home run.

In June 1951, nearly two decades before the ritual became a thing at Chicago’s Wrigley Field, the baseball New York Giants asked all spectators to “return foul balls hit into the stands to forward to our troops in Korea,” the Brooklyn Daily Eagle reported at the time.

The game-used balls stamped “FROM A POLO GROUNDS BASEBALL FAN” and signed by the likes of Hollywood starlet Laraine Day, wife of then-manager Leo Durocher and kids from Staten Island, are up for auction.

The surplus “soiled, scuffed” baseballs are one of 11 lots — including ticket stubs, programs and a game-used Willie Mays bat valued at “$10,000-plus” — from the collection of former Polo Grounds superintendent Jim Trainor up for bids. In contrast to the Mays lumber, the balls can probably be had for “between $500 and $800,” the auctioneer said.

When the Giants split for San Francisco in 1957, Trainor sadly stayed behind, son Kevin, 72, told The Post.

“When they left, it took a part of him,” Trainor said.

The Maryland-based Huggins & Scott Auctions is staging the online sale, which runs through May 28.

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