This is a modified no resistor rotor. We start with a new Rotor, remove and replace the resistor with a solid piece of copper wire. It can be used on all 123ignitions. The intended purpose is to lengthen the life of the cap and rotor when used in conjunction with CDI units like Winterburn and MSD CDI boxes.
Why No Resistor?
"The rotor resistor was an early attempt at radio noise suppression. Same role as the resistance in spark plug wires and the spark plugs. No other reason. The resistor actually causes the secondary side voltage to rise slightly higher by a few percent before spark breakdown so the voltage overshoot is higher. Without the resistor there is less voltage stress on the coil, distributor cap, rotor, and spark plug wires. In addition, once spark breakdown does occur that added resistance will reduce the spark energy due to the current limiting it provides. For CDI use, the resistor should be removed and replaced with a piece of wire, or eventually the higher power of a CDI will cause the resistor to go open-circuit" - Contributed - Frederick C. Winterburn
Additionally the OEM Rotors just fit better due to the parallel bore and step design to fit the 123 shaft. See these pictures below. The one on the left is the standard aftermarket and is a tapered bore. The center and right are the oem and an attempt to show you the parallel and stepped bore which give a very secure fit.