Internally Balance Cast Crank?

I've got a crank out of a 73 340. It should be and looks like a cast crank from everything I've read. However, it appears to have been internally balanced with extra holes in one of the counterweights. It also rings like a bell when I compared it to a known 70 340 crank. Any way to figure out what I have. There is no casting numbers on said crank where the 69/70 crank has the casting number. I can take some pics if needed.