I am getting ready to put my Prairie back together. I decided to order new rotors for the oil pump because I figured I might as well while I'm in there, and they're only $5 bucks each. Anyhow, I see both rotors have a dot machined into them. Service manual does not say anything about them, nor does the photo show them. The rotors appear symmetrical, and I can't see how one side would need to be installed in any certain direction. Anyone know the story on these dots?