Going way back, some Johnsons had “maverick spark suppressors” to prevent stray unwanted “maverick” sparks. On those, the secondary winding was grounded through the spark suppressors instead of directly to the armature plate. The secondary winding was grounded through the suppressor, but not the primary because it would not work if the primary was not directly grounded. Thus the reason the primary and secondary were individual wires.
BTW, the secondary was not truly grounded, but only close, via the suppressor. The spark current had to jump that almost-connected gap, resisting stray, unwanted sparks.
No reason the 4-wire coil cannot be used with non-suppressed mags, so no reason to make another part number for the non-suppressed mag.
EDIT: If you attempt to ohms-test the SUPPRESSED secondary winding by connecting your meter lead to the mag plate, it will indicate an open secondary. On those mags you have to connect the meter lead to the secondary ground WIRE, not to the plate.
