Dan,
Your understanding of the resistor/no resistor is correct. The harness/loom resistor that GM settled on in later years replaced the external ceramic block type of the early 12 volt days. I never heard it said by anyone at GM, but I think harness resistor wire was cheaper, and maybe more durable than the ceramic block. Otherwise the voltage-dropping function is the same. I seem to remember that early Ford 12 volt systems had an external ballast resistor that used fairly fine wire for the resistive element, and was contained in metal housing. It was attached to the ignition coil, or close to it. I remember that type being prone to failure (but now we're in the realm of foggy memory and ancient history). The ceramic block that GM and Chrysler used was an improvement over the Ford type, but they were still more prone to failure than the harness type.
Some Chrysler products, and I think a few Delco-Remy ignition systems for non-GM appications, used a coil with internal ballast resistance, but those had a tendency toward hard starting in severe weather/low battery conditions. I think that was what led Chrysler (Electric Autolite/Prestolite) to switch to an external ballast resistor and a non-resistor ignition coil. The later Chrysler external ballast resistors had two resistor elements side-by-side, one for the ignition circuit and the other for their electronic voltage regulator (I think). The two elements had different resistance values, and it was a common mistake to connect the wires incorrectly when replacing the dual resistor part.
It was also a common mistake to replace a coil with the wrong type- non-resistor coil with a resistor coil, or vice-versa. Sometimes it took a while to sort out why the car was hard starting, seemed to have short spark plug life, or point life was only a couple of thousand miles.
Hope this helps.
Dean
