The coil ground is for the primary coil to the breaker points. The secondary (High Voltage) coil is grounded through the spark plugs being screwed into the cylinders. If one side is missing it is because the spark has found a lower resistance to ground (Engine block) than through the spark plug. Look for a crack in the plug wire insolation, usually where it is clamped to the magneto plate. . . 😉
