Bosch makes a 6-volt version of the blue coil that is silver (unpainted steel). It puts out 40,000 - 50,000 volts on 1.2 ohms primary resistance - vollig ausgezeichnet!
Just keep in mind that actual coil output voltage varies. A coil will only put out the voltage necessary to jump the spark plug gap. When the points open and the primary field collapses, the voltage induced in the secondary windings rises until it is strong enough to jump the plug gap. Thus, if only say, 5,000 volts is needed to jump the plug gap, 5,000 volts is all the coil produces. When you go to wide open throttle, cylinder pressure rises and so the voltage requirement also rises, maybe to 15,000 or 20,000 volts. If the plug gaps are badly worn, the coil secondary wire has a break, if you have a high compression head or a turbo/supercharger, voltage requirements will go higher. But I don't think a Model A would ever see the need for a 50,000 volt coil.