I agree a little with Kevin but they could have replaced it and used the 6V wiring. If that cloth wire is integral to the motor, it's 6V.
I would think a 6V run on 12V would almost be a hovercraft.
You can test it for 20 minutes using a 7A-10A fuse. Probably should have the squirrel cage on it to create a little load. The speed and the amp draw may give a clue. None of the above is totally definitive. Otherwise, as you said, get a 12V motor.. Which of course, is not all that easy sometimes. Hard to search internet "replacement 12V fan motor for a 47 Chevy truck." Except possibly in
Stovebolt Search 
You CAN find one by diameter and length of housing....... diameter, length and shape of shaft...... Give us that info and a couple good pics of the motor if you want, and we will find. You probably should replace it during a refurbish anyway. PITA to get to again.
There are methods to electrically and physically determine, but beyond most folks capability and not 100%.
