Is there a way to tell which 12a rotor is which? Is there even any difference between the two? I know that they are different part numbers but can't see any difference, and can't imagine any reason for them to be different.

While we're on the subject - any way other than weighing them to tell the 79-82 rotors from the 83-85?

(I am rebuilding an engine that needs a front rotor, and I have a good rotor that I had been using as a bookend, but I don't know which end or which year it came from.)

Thanks!
Steven