Two kinds. The ARD belt drive system based on the Fairbanks-Morse mag head and mounting in front of the cylinder. The "undercover" type is the more modern one and uses an external coil. I have used the early ones and they are ok, but have no advance, so idling and starting is at full advance. I wouldn't spend much time at idle with the mag. Some people think that the mag blocks airflow to the right cylinder.
Both seem to be very reliable.
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