This solid rubber #15 drain plug is heavy duty to retain water in your Petersen automatic waterer.
Found in Petersen concrete waterer models:
44C, 74H, 74S, 75, 84S, 94S, 120S, 120H, 144, 150, 150C, 150S, 175, 225, 250, 300, 350, 400, 450, 474S, 484S, 494S, 500, 650, 700, 800, K-9, Thermal Pete 1 x1, 2 & 4