The voltage IS a little less than 9 volts after the ballast resistor. It is also 6 volts at the points! Just like 6 volt systems, nothing changed. But to delve into it deeper, you can take your stamped 12v coil and put it on your 6v system and it will run the same and visa versa. This has to do with mathematics that are irrefutable. No slams on anyone else, no malice on my part, just reporting the facts.
Spend a few weeks studying this with a bunch of different coils from different era's and measure voltage and resistance. Then see the differences in power and you will have no choice but to come to the same conclusion. I was told about this from an ignition system pro when creating the HEI and then studied it myself. It's not a big deal guys! You still want the 12v stamped coil on your 12v system because its newer and made of better materials. But that's the only difference. I still have all of my coils lined up with tags on them from the testing.
