More disc brake questions

I opened up my rock auto master cylinder boxes today. There is a reciever groove in the back bore of the master cylinder piston for the pushrod O ring on both master cylinders. Also for the life of me i cannot see what is remanned on these. Neither of the body castings are chrysler castings they are brand new aftermarket castings, and the reservoir plastics look new as well. One body casting aluminum which i will keep for my car, the other casting is cast iron. This adds a little weight but is still lighter than a full cast iron unit. I will keep that one for my sons car.