Nothing. They didn't say specifically, but it looks to me like its probably just where the hole is, in the casting mold, where they pour in the molten metal.
They were pretty adamant though, that they definitely aren't balanced, and they didn't see any reason why you would want to. The guy I talked to did seem to know what he was talking about. He did agree there might be some small benefit to balancing them, if the motor was going to turn 6-7,000rpm. Obviously that's way above what a stock (which is what those rockers meant for) 4.0L-OHV would ever see.
So, my plan is to smooth/polish the rockers, along with removing that pad. I MIGHT try to get them "static" balanced, by putting them on the shaft, and seeing if they "flop" one way or the other, or just stay centered. I probably won't put too much effort into it though.