I saw this mentioned at the Usenet group rec.radio.shortwave.

http://www.monitoringtimes.com/mt-bball.html

QUOTE
This year The Baseball Listener’s Guide lists more than one thousand AM and FM radio stations throughout North America which air Major League baseball games throughout the season. The purpose of this guide is to allow baseball fans throughout North America to tune in to America’s national pastime as has become the tradition since 1921 with the first broadcast of a Major League baseball game by KDKA, Pittsburgh.