DC tac hook up..?..

I've had this tachometer for a number of years and I posted about it when I first got it and there was a few different opinions on how to hook it up, but I never tried. It's just been sitting in my glass case for years now and I would really love to put it in my car... I posted this in the race forum hoping some old timer might know what's going on with it as it only has two wires??
Of course I got people just saying hook it to the coil and see what happens? I'm not looking to hurt it or hurt my car trying wild ideas.. one train of thought that it was a Magneto triggered tack?.. I just noticed it says Sycamore Illinois USA.... first has anybody seen one like this? And second how in the heck do I hook it up... Correctly?..
I appreciate everybody's experience and opinion...