Help! Isolator?

[78c20]

I have a 1978 Chevrolet C20 here in the United Kingdom that is probably ex-USAF.
I have recently imported ( from the USA ) a 350cid small-block with a few performance goodies fitted. Now the problem is, located on the firewall is a small ( approx 3x5inches ) finned metal box marked battery-isolator. The box has three terminals on it. Are these three terminals ie 1 for feed in, 1 for return to charge batteries ( truck has two ) and one to the lamp in the cab?
Can anyone tell me what this thing is/does and do I need it?
How should this be wired into the wiring-schematic?

( original engine was a straight-six if this helps )

Kindest Regards from the UK.

[Russell]

That thing is for your dual battery setup. If I remember correct, the way they are to be wired up is that the one wire comes from the alternator, and the other two posts go to either battery's positive post.

[bagged74]

Originally Posted by Russell

That thing is for your dual battery setup. If I remember correct, the way they are to be wired up is that the one wire comes from the alternator, and the other two posts go to either battery's positive post.

that is correct.

that thing is basically just a big diode, to keep the batteries isolated from one another, but allow them both to charge.