I will attempt to explain what I know about this in one post.
The 139 tooth flywheel will ONLY accept a 6 volt starter. Since the gears are not interchangeable between the 6 and 12 volt versions, and you will get grinding, resistance, and uneven rotation with the wrong one, you must know the number of teeth on your flywheel. 139 teeth = 4 teeth per inch while 168 teeth = 5 teeth per inch. The pitch of the teeth on the starter is different to accommodate this. You just don't want to get the wrong one and its ALL based on the flywheel.
Now, the reason its a GOOD thing if you have a 6 volt flywheel even when converting to 12 volts is because 6 volts requires TWICE the current as a 12 volt system so the wiring, windings, everything about that 6 volt starter is beefier to handle the extra current. Since they over-engineered this particular starter, you 12 volt guys can benefit. BUT, the reason you hear everyone say "Do not spin the starter for long periods of time" is due to the Higher voltage creating more heat inside the motor. This has to do with 12 volts being twice the wattage (power formula). If you run the starter a normal amount of time it will last exactly the same amount of time on 12 volts.
You know you have a 6 volt starter if the teeth on the flywheel say you do. The only possible exception is someone cobbling together a 12 volt drive gear in an attempt to use a 6 volt starter on the 12 volt flywheel. This is not a mainstream thing and although of course it can be done, its highly unlikely you have one like that.
I know this is confusing. So do this:
1) Check your flywheel. 6 volt ring gear or 12 volt?
2) Order the appropriate starter. If it has 4 teeth per inch, you need a 6 volt starter. If its 5 teeth per inch, you need a 12 volt starter. Everyone should know the number of teeth on their flywheel because it will make your life so much easier when on the side of the road needing a new starter.
I guess for me, the problem with looking at the starters part number would be if by chance someone did a modification to it. Then of course since NAPA wants your old starter, you can look at the gear and see if they look the same too. You are doing fine!
BTW, many people with the newer style 235's have 6 volt flywheels. There was no reason to get another flywheel or starter and saved them money. So you just don't know what you have until you do the inspection.
