Williams/Bally black plastic opto interrupter.  

Used on the following Williams/Bally machines:
Attack From Mars
Cactus Canyon
Champion Pub
Congo
Corvette,
Dirty Harry
Demolition Man
Flintstones
Indy 500
Indiana Jones
Jackbot
Judge Dredd
Johnny Mnemonic
Junkyard
Monster Bash
Medieval Madness
NBA Fastbreak
No Fear
No Good Gophers
Popeye
Roadshow
Safecracker
Scared Stiff
Star Trek The Next Generation
Shadow
Theatre of Magic
Tales of the Arabian Nights
World Cup Soccer
Who Dunnit

Williams/Bally reference #03-9001.1.