Which pins to measure flah trigger voltage?

[stuistead]

I have a fairly old (about 10 years) Jessop 300AF - Mi Flashgun (A UK branded unit that I had for a variety of Dynax film cameras) that I want to use on my 5D. I know that I can only use it in manual mode, but I want to be sure it won't blow the internal circuitry.
Which pins do I measure the trigger voltage across on the bottom of the flashgun? As a reference point, lets assume you are holding the flash so you are looking at the pins on the base of the gun and the flash head is pointing upwards.