I think the old style coils were designed and built on the technology of engines that operated on 6 volts continuously. 12 volt starting systems came about because of the lower line losses and highter efficiencies of a higher voltage. most gauges will operate from 3-48 volts fine but they used a constant voltage regulator to keep them accurate. The Coils on the other hand could operate only briefly at 12volts before overheating and meltdown.