Ground. Usually it was an open braided cable and ran to the following places:
a) Start at the RF noise filter (as shown in the picture)
b) run to the back door kill switch metal plate
c) run to the frame of the monitor
d) run to the bolts on the inside of the coin door
e) run to the bolts on the metal control panel
f) optionally run up to the flourescent lamp fixture metal frame
All it is is a ground wire to ensure that any wiring faults will quickly be caught and head to ground instead of through something else (namely you)
PS: You should probably remove the 25 cent piece that is loged under the tab of the noise filter before THAT shorts something!
