You have to look closely to see it in the background of the film footage. But as Lou Gehrig, on July 4, 1939, was delivering his “luckiest man on the face of the Earth” address in Yankee Stadium, perhaps the most famous speech in sports history, it’s off there in the distance—an all-capital-letters sign on the outfield wall just to the left of the scoreboard: “Betting prohibited.”

Likewise, when Don Larsen was pitching the first and only perfect game in World Series history on Oct. 8, 1956, there was a sign on that same outfield wall. You can see it in photographs taken from certain angles—slightly reworded in the years after Gehrig’s speech, but still in all-caps so no one in the ballpark could miss it: “No betting.”

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