ballast resisters

"Regulation" is correct, but it is only PARTIALLY the story. The advantage "back then" of using a resistor / coil combo instead of a "full voltage" coil is, that when starting, the battery voltage is drug down. By using a resistor, then they could use a bypass circuit for starting and get the voltage "back up" during crank

IE what you have is

Fully charged battery, nominal 12.6

"Running" down the road, nominal 14V

But STARTING around 10.5, and on a very cold day and if the battery might be down, "less" maybe down 9V or even less

So by using a resistor and coil combo designed to run on 10-12 V nominal, you can still have good supply voltage to the coil when CRANKING

"Everybody" did this, pretty much, Ferd, GM, Ma, and AMC. All used a resistor and a start bypass setup. Fraud, AMJ, and GeeM "did" the bypass with the starter solenoid. "Ma" chose to use the ignition switch.

"Everybody" could have chosen a coil built for "straight" 12V service. Many tractors and industrial machines like welders did things this way. Most these had lower compression ratios which start easier with low voltage than do hi performnance, high compression engines.