Tech-S/W tach wiring ?

I have an 8,000 rpm Stewart Warner Tach and can't figure out how to wire it . It has a baby blue plastic housing and three threaded posts sticking out of the back and nothing is marked . One terminal has a bus strip that runs over to the lightbulb hole that looks like a ground to me . That leaves two others . One is just a stud sticking right out of the case and the other has a short shoulder where it comes out of the case . I know this description is pretty lame . Can somebody set me straight ? Thanks.

 

no colors anywhere huh?

 

There's none I can see . There are no wires , just the posts sticking out .