Old rubber cushion squares on 1970s Gottlieb glasses
1970s Gottlieb heads used small 1 in black rubber squares between the backglass and the light board; these often stick to the paint and leave faded squares. If they have not stuck yet, replace them with a non-damaging cushion placed at the glass edge. If they are already stuck to the paint, leave them, as pulling them off may lift paint.
Systems
Related tips
Source: Pinball Machine Care & Maintenance, Bernard B. Kamoroff (Bell Springs Pinball Alley)