Could this be a ballast resistor? The original coil should be labelled as to whether it is a ballast coil or not. With a ballast system, the voltage to the coil is reduced by the resistor for normal running. The ballast coil is designed to give a normal spark with less than 12 volts applied. But there is usually a second feed or some means to by pass the resistor when you operate the starter, so that the coil then gets full battery power to improve starting.
Google ballast ignition systems for more detailed explanation.
