Tachometer wiring , Help needed.

 I have been asked to fit / wire  what appears to be a period accessory tachometer to a 79 Triumph 1500.

Apologies for forgetting to take pictures,  the tach does not appear to have a make or brand name printed on the face or cowling., a quick look reveals  4 wires soldered onto the internal circuit board, the original wiring diagram instructions are long gone. 

I assume that the  4 wires are  live feed and earth, and  connections to the coil, I am uncertain which wire goes where, could anyone here please enlighten me? 

The wiring colours are

Black,
Green,
Red ,
White,

 Thanks.