I believe it is just a newer style 500 piston. Probably aftermarket as well. I saw in your 85 build thread that you went to the Unadilla rewind...where are you from? I live up in the fingerlakes area of NY.
I am from NY originally, but I live in MI now. I still have family in the Downsville, NY area. I will be at the Rewind again this year. It is one of the best vintage race events of the season. Definately one to attend. Your bike is eligable for the promoter's Retro class, but not AHRMA (yet). Hopefully, the Ultima rules will change for 2017 and allow the non-powervalve, front disk brake KXs.
Cylinder and head are looking good!
Consider the compression release mod for the head. It's a good idea, even if you can kick it over with out it. Having the release puts much less stress on the kick start mechanism.