Dirty Slots,
if the item you are talking about is in the circle in the photo below, it is the adjustment for the handle return buffer pump and the screw controls the air that escapes thru the valve which controls the speed at which the handle returns to prevent back slap of the handle!
The nut just locks it at the setting selected!
You can easily place a bulb behind the award, the marque and below the reel window using a door bell transformer for your 6-8 volt DC power for the bayonet bulbs!
Not original power source but for looks it can be done!
Bona Fortuna!
MONTI