Tach help please!

I have an RAC tach from eBay and I need a little help with the wiring.

It has four wires: blue, yellow, black and green.

Blue and yellow go to the light only. Black and green connect to the tach's main circuit. I can't see any place where there could have been a now missing red wire.

Do two wires mean it's a current triggered tach?

Anyone got one of these or a circuit diagram? Would treating it like a current triggered tach and wiring the black and green the wrong way around kill the tach? This is going to be driven of an MSD ignition with a tach adapter.



Here're some pictures:

 

Green wire connects to coil + terminal and black is usually a ground. A quick touch of wire to coil will not harm tach.

 

wouldn't that be to coil - ?

 

Yes. Typically:
Blue- lighting circuit
Yellow- 12v switched
Green- trigger (coil -)
Black- ground

 

WOW! What timing. I bought a similar RAC tach awhile back, and just brought it in the house last night to begin restoration on it. I was looking at it this morning wondering where I'd find out how to wire it. My tach also has a red wire with the green and black. Any idea where that one goes?
I LOVE THE HAMB!!!!!!!!!!!!

 

Red - keyed power
green - trigger
black - earth.

Typically as stated before

 

OK, My problem is that I don't have a red wire or any sign of there ever having been one. Two go to the light (yellow and blue).

So I've just got two wires to operate the tach - green and black. Does this sound just plain wrong or does anyone know what I've got?