in the 1980's, the white sox actually used a light on their scoreboard to tell the batter if it was a fastball or breaking pitch. A member of the White Sox organization would sit in the manager's office and watch the camera that had a view of the catcher from center field and would turn the light on or off to indicate the pitch. Simple, effective and incredibly shady all wrapped into one. https://www.espn.com/mlb/columns/story?columnist=kurkjian_tim&id=1857661


The most damning of all may be the 1951 Giants that admitted to having a buzzer into the dugout to alert players to what type of pitch was coming.

(https://www.newsday.com/sports/through-the-looking-glass-giants-used-telescope-buzzer-to-steal-and-relay-signs-during-1951-miracle-of-coogan-s-bluff-1.426471)