Inside of the 1968 GE M-100

Top housing is pretty basic. The only wiring is the PC socket and it's leads and the plug for the ballast door and its wires, which all connect at the terminal block. Incoming hot leg 1 goes to the far left, incoming hot leg two goes to the second terminal, and common/neutral for the 120V PC socket goes to the far right. Third terminal is for connecting the load side of the PC to the ballast plug.

As you can see, the socket is attached to the ballast door. This would be a convenient lamp tester if the ballast wasn't 240 volts.

Note the black cylinder mounted near the lamp socket. It's connected across the two terminals of the lamp socket. No idea what it is. I imagine some sort of surge protection? But wouldn't it be on the line side of the PC socket before EVERYTHING? Why protect just the lamp? After all, the lamp is the expendable component in this scenario. Makes me think it's for something else. I don't think it's a capacitor since again, it would be ahead of the ballast, no?