At the Polo Grounds, the Giants drop an 8 – 3 decision to the Cubs. New York rookie Les Layton, in his first major league at-bat, hits a pinch homer in the 9th off Johnny Schmitz.

On May 21, 1948 — At the Polo Grounds, the Giants drop an 8 – 3 decision to the Cubs. New York rookie Les Layton, in his first major league at-bat, hits a pinch homer in the 9th off Johnny Schmitz.

Source:
Baseball Reference May 21