Quoted from Cdemuro:They are used to align the flippers. Point your flippers to them and tighten them down. Most EM games by all brands I have had used those as references.
I've never seen them on Gottliebs, Williams, or any earlier Ballys I've worked on. They would sure come in handy though. Lining up flippers evenly and at the correct angle can be a tedious endeavor.