Take a look:
100_1634.JPG
100_1635.JPG
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:
http://www.homedepot.com/p/Unbranded-6-1-3-4-in-Phillips-Round-Head-Wood-Screws-40161/203086547
Picture is way off (looks like a #10 or something) but it's for reference really. Seems about the closest I can get. Thanks!