360 crank question (weld in crank)

The guy I got it from Knows nothing about it,it was in his dart when he bought it. He has the auto flywheel that was on it and I have a 4-speed. I tried to call him to look at the flywheel but he is not home.What is the diff between balance and not balance auto wheel? I just looked at the balancer,it is a solid piece with a big groove hollowed out on one side. I would post a pic but wife is not home.Thanks-all.