Internal and external balance.

The factory externally balanced engines to save money. It was usually done with less expensive cast cranks, which use a lighter material than forged steel cranks. The counterweights are lighter so the extra weight to counter the weight of the rods & pistons has to be made up with extra weight, which is hung externally on the ends of the crank to balance everything, as Rumble described.

Race engine are internally balanced because it's suppose to make the rotating assembly more durable and reduce flex a bit. But the extra heavy Mallory Metal slugs pressed into drilled counterweights to do this are expensive.