Take a look:
I believe it to be a #6, 1 3/4" length wood screw with a round washer head.
It's used to screw down plastics to the playfield that are standing on those metal spacers.
I can't find them on PBL, Marcos, Home Depot, Lowes etc.
The closest I think I can get would be this:
Picture is way off (looks like a #10 or something) but it's for reference really. Seems about the closest I can get. Thanks!