Mini Message Center - Hour Meter?

I picked up this gauge at a flea market for $1, hoping to figure out what it is. The part number comes up with nothing, but I did find a similar looking label on another gauge for a Peterbilt in an Image search.

Anyway, I definitely found the Ground pin, but can't figure out which is 12v. I tried applying voltage to the Positive lead on a few large 16v capacitors, but most don't do anything, and one of the smaller ones turned on the screen but all segments were lit, so no numbers or letters.

Is this just a word display for displaying codes? My first guess is that it's an hour meter judging by an old ebay listing, but who knows if the seller actually knew what it was.

 

Where can I find more info about the different pins? What's it used for?