Its a special relay, that closes the idle fuel cut off valve, when the revs are above a certain level (about 1400) and the throttle is shut. Its a fuel saving device.
Eg, on cruise, lift off, the fuel cut off activates, stops sucking in excess fuel, until the revs drop off enough, then the carb effectively gets 'switched on' again.
Thing is, I've only ever seen it on 1.4s, I'll look in my 360 book in the morning.
I have one in mine, and the number is 209 472, but mines an 89 340 1.4. I had a quick scan through the haynes, but I cant find it so I have no Idea what it is supposed to do