Gottlieb pop bumper trim platters
Gottliebs after 1965 have round clear mylar trim platters around the pop bumpers. Peel them, remove the glue residue with an adhesive remover, and clean underneath. Replace them only where wear is bad; adhesive-backed ones are better, because loose ones shift as the ball crosses them and grind the paint they were meant to protect.
Systems
Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)