When carmakers changed over to 12 volts, they continued to use 6 volt coils. They would use a ballast resistor in series with the coil so as to limit the current, lest the coil get too hot or the points burn. It wasn't unusual to bypass the resistor while cranking the engine, which would give a hotter cranking spark. Some used ballast resistors and some used resistance wire. My old '67 Cougar used resistance wire. Eventually they started building the resistor into the coil. The resistor is internal to the coil and you can't bypass it. Some coils were labeled as "12 volts when used with resistor." Well duh, I suspect this was more a marketing way of selling a 6 volt coil. In theory you can double the primary turns in a coil and get the same input results as using a resistor, but that will halve the secondary voltage. You can double both turns, but in theory your condenser won't match. I suspect this is what they've done with these "high energy" coils, but you're more likely to see those matched with an electronic ignition system.
The bottom line: You don't want to use a 6 volt coil in a 12 volt system without using a resistor. You'll start burning out points and overheating the coil. At the opposite end you don't want to use a true 12 volt coil in a 6 volt system because you won't be generating a very hot spark.
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