Williams/Bally/Stern blue "START" button.
Comes with switch, lamp, and mounting nut.
A great mod for many games such as Twilight Zone, Tron, Roller Coaster Tycoon, No Good Gophers, Monster Bash, Johnny Mnemonic, Iron Man, Funhouse, Whirlwind, Avatar, Batman The Dark Knight, and many others.
Stern part #500-6388-44.
Bally Williams part #20-9663.