If you look at the disrtubutor gear itself, and the relationship of the gear teeth to the rollpin, you'll see that the rollpin hole is in a different location in relationship to the gear teeth. Many years ago (in my early days), I worked on a '65 Malibu SBC that would missfire and stumble. The end result was the gear was miss-indexed. On a HEI ditributor, the magnetic pole piece will be out of time, causing weak spark (primary ignition breakdown). Many years ago we ran a test on a Sun distributor machine and found that the indexing was important on primary ignition (the actual results as to spark are long time forgoten - believe it delt with reserve spark/ gap in distributor causing missfire between cylinders). We always looked for the index mark whenever disassembling a distrubutor and all GM's had the mark (if they didn't, the rollpin was off-center so the gear would go on one way ). I always figured the General wouldn't havve spent the money putting the index mark on the gear if it wasn't needed, so why fight it.