From Alan Schwarz's The Numbers Game: Baseball's Lifelong Fascination with Statistics:
To spread the gospel, Chadwick invented his own personal scoring form in the hope that it would become standard. The grid extended nine players deep and nine innings wide, each box housing a player's at-bat as he came up in turn. Each outcome was coded with either numbers, for the fielder who handled the ball, or letters, to denote a fly ball or the like. (Many of those letters were chosen as the last of the events they connoted, such as "D" for catch on bound, "L" for fouball and "K" for struck out, the last of which has survived to delight generations of kids as they first learn to score.)
And no, "K" doesn't stand for Koufax.
Posted by John Fleck at December 28, 2004 07:32 PM
When he wrote his name in capital letters in the record books, that "K" stands out even more than the O-U-F-A-X.