I just finished the wiring on a 41 and did not have an extra wire from the distributor. Is this wire on the same post as the wire that goes to the - side of the coil? If so it must have been added for a tachometer. If it is a wire attached to the body of the distributor then it must be an added ground. The reversing switch (i did not use) is located at the starter and is connected to the + side of the coil. It sends full battery voltage to the coil when the starter is enguaged. You should have only 1 wire from the distributor (except spark plug wires) and it should connect to the - side of the coil. Any other wiring to the distributor was added after market.
Not to make things more difficult but are you using a 6 or 12 volt battery? If you are using 12 volt you will need 12v points (NAPA has them) and a voltage droping resistor that hooks to the + side of the coil then you hook up the battery (ignition switch) wire to the resistor.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"