I suppose that possibility could be tested pretty easily. Is the weight of the counterweight equal to the weight of the two set screws? The rotational timing would be wrong, and I think the counterweight is probably considerably heavier than the screws.
The counterweight mounts exactly opposite the keyway, and the set screw that keeps the key from moving. This seems to fit the idea that its purpose is to balance out some kind of effect of the key and its set screw. Hence the speculation that it is to compensate for clutch bore damage caused by the key fouling on the thrust bearing.
