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National Baseball Hall of Fame and Museum ⚾ on Twitter: "Lou Gehrig's #4 was the first number retired by an @MLB team, and it opened the door for other teams to follow
The Yankees Retire Paul O'Neill's Number 21 - The New York Times
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The New York Yankees Could Run Out of Jersey Numbers After Retiring Them - What Happens Next? - EssentiallySports
Yankees retired numbers: A look at all of the jersey numbers players cannot wear in the Bronx - CBSSports.com
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