Yellow "Start" button.
Comes complete with push button, lamp assembly (including bulb and switch), and tightening nut.
Used on the following games:
Cactus Canyon
Champion Pub
Cirqus Voltaire
Junkyard
Monster Bash
Medieval Madness
NBA Fastbreak
No Good Gofers
Safe Cracker
Scared Stiff
Star Trek; The Next Generation
Williams/Bally reference #20-9663-16.