How can I tell what year 12a I have and what are the differences between all the years? Also, how can you tell the difference between the earlier 12a rotors and the lighter 83-85 12a rotors? Thanks, I have two torn apart right now and they both came out of old school cars but look like later model first gen blocks, I think. Thanks

 

As long as you remember which flywheel came off which engine you can use the flywheel.
http://www.mazdatrix.com/faq/flywheel.htm

 

Do the side plates have a Y cast into them?