From http://modelabasics.com/Ignition.htm#polarity
How The Coil Works
The coil itself does not care which way it is hooked up, it will be just as efficient either way. On the other hand the spark plugs do care. They prefer the center electrode to be negative and the end electrode to be positive. The reason is electrons like to jump from a cold surface to a hot surface and the center electrode of a spark plug is hotter than the end electrode. By correctly establishing the coil’s polarity the voltage required to jump the spark plug gap is 15 to 30 percent stronger which may or may not be evident on the performance of your ‘A’ depending on the condition of the ignition components. Oddly enough the large amount of voltage created by the coil is not affected, positively or negatively, by the mere 6 volts (or 12) of the battery. Therefore; positive ground or negative ground of the battery is not relevant.
Tom Wesenberg Tip:
"To check coil polarity hold a pencil lead in the spark path with the plug wire about a third of an inch away from the distributor terminal. You’ll see a FLARE on one side of the pencil lead and this flare should be on the spark plug side of the lead." Tom suggests to hold the pencil in the middle or even slide the pencil into a plastic tube to avoid an electrical shock.
If the flare is on the plug wire side. Switch the two wires on the coil.