Dear Dan;
The external resistor used in the 50s, reduced the voltage from +12V to about 6.8V when it warmed up. The coils used then were basicaly the same as the older (6V) ones which ran at 6.8 to 7.2V.
This allowed the coil to have full voltage in the 'start' mode, lowering it in the 'run' mode, which added to point life.
Later (60s) the resistor(s) were "internalized" or done away with & the coil primary winding made heaver, where some MFG used a 'resistor wire' in the primary circuit.
Happy trails.
