Given that the post screws with hex heads don’t use washers—and have roughly the same surface area as the slotted screws for securing the post evenly—I can’t see a mechanical reason for the washer. It might be a cosmetic flourish. They polish up to a bright shine and add some bling, which is something the posts lost when they moved to plastic in the late ’60s. To my eyes, the post looks a little naked without that shiny washer.

Did you clear the playfield with lacquer?

I’m reassembling a playfield over here in my shop, so I held up a hex-screw post and slotted-screw post. Here is the reason for the washer: they help level the plastics because the slotted head is shorter than the hex head. The washer adds a little extra height to even things out.

I just buy new stainless stee washers from HD for my pf refreshes

Which Minwax Polycrylic did you use? Satin or gloss?