Technical Tip:

Identifying Ford Starter Motors: 12 Volt vs. 6 Volt by Bob Haas

Last December I was looking thru my stock pile of starter motors for one to install on my car. I did not know which one was 12 volt or 6 volt since no tags were on the motors. So I called a few Crown Victoria Club technical advisors and club members, but no one knew how to tell the difference.
We all agreed that a 6 volt would work on a 12 volt system, but the RPMs of the 6 volt starter will be twice as fast. The higher RPMs can damage your ring gear and will run down your battery faster if you crank it for a long time. (If you do  use the 6 volt on a 12 volt system, do not crank it for a long time.)
I took two of my starters to the local alternator shop that has rebuilt many starters and generators for me. His name and phone number are: Dana 248-437-6082. Dana pointed out the difference between the 12 volt and 6 volt starters.
If you take the band off of the 12 volt starter and look at the field coil, the copper band that goes to the terminal stud is about the thickness of a credit card whereas on a 6 volt starter the band is the thickness of three credit cards.
Also, a 12 volt generator has the adjustment mounting boss toward the front of the car whereas the 6 volt generator has the adjustment mounting boss toward the rear of the car.

                 
Now that you have this information, go out to your shop and mark all your starters and generators. Also make a copy of this and keep it with your parts!