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Just traded for a 57 cj 5 with a 6 v system. Previous owner used a 12 v battery on the jeep. What kind of damage would this have done to reg, starter, generator? How do I test regulator and generator to see if they are working properly?
A multimeter is going to become your friend by the sounds of it. All the bulbs will likely be shot too.

Are you sure the system hasn't been converted over?

The 6 volt stuff is much heavier than the 12 volt stuff, so things like windings are fine. There is surprisingly little that has to be changed from what I have read with some folks out there actually rewinding some of the older stuff for 12 volts.

Best you can do is put a 6 volt battery in and see what happens. Even a $10.00 meter will tell all.