In those days, one wire of the relay would connect to the generator (as relay ground) to ensure that the starter could not be engaged one the generator is turning (producing power, the engine is running). So the generator armature is the ground for the relay, allowing the relay to be activated. When the generator is producing voltage, it is no longer ground. Both sides of the circuit go to battery voltage when the gen is turning (at speed), thus no current flows through the relay. In later years (1957, 1958) they used a vacuum switch to disable the starter relay so it wouldn't engage while the engine was running, instead of grounding the circuit through the gen armature.