I haven't had time (or maybe the ability) to figure out the ohm readings, but it looks to me like that add-on is some sort of resistance bridge to make the ammeter react like a voltmeter.
From the configuration of the terminals (I assume it came to you that way) you'd connect power to terminal 1. Is that the one that's marked "IN"? I would probably put a reasonably sized fuse (maybe 10A) in the power feed to it.
I didn't see a picture of what the front of the gauge looks like. Has the face been converted to something indicating volts instead of Charge & Discharge? I'm wondering if the needle would just sweep from center position to some position on the right if 12V were applied to terminal 1.
