Small "bulb hole" is the indicator. There is a light bulb under it that flashes when the turn signal is on. It flashes either left or right. That "thumb wheel" rubs on the steering wheel hub and cancels the turn signal. The parts inside are simple and straightforward so no worry's. Take it apart and clean it up. Bet it still works if you replace the light bulb and maybe the flasher unit. Replacing the rubber on that wheel is gonna be an issue that I have no solution for. As long as it is not cracked and crumbling I would leave it alone. Think 6 volt for light and flasher if you still are at 6 volts. If you are at 12 then replace the light and flasher with 12 volt items and you are good to go. You might need a second set of tail lights for the rear if they are missing. Was an option or aftermarket install not a factory thing. I added bullet type connectors to my unit when I took it out to clean and fix. Made it easy to put back into service. Just hid them under the dash so no one can see them.

Steve H