Who did Grayson strike out in his last at bat in the minors?

In chapter 25 of "Maniac Magee," Maniac has been staying with Grayson who was a Minor League baseball pitcher.  He is constantly telling Manaic stories about his days as a player.  on page 94 we read Grayson told"

"Funny stories. Happy stories. Sad stories. Just plain baseball stories.  The happiest story being the one in the Minor Leagues, before he went up to the New York Giants and immoratality.  Well, it was ol' Grayson himself who had the last crack at Willie Mays, in the ninth inning of a game with Indianapolis - and what did Grayson do?  All he did was set the Say Hey Kid down swinging - on three straight curveballs."

So Grayson had the happy memory of striking out Willie Mays right before Mays was called up to the Major Leagues.