what part is this called?

Hey joshua;
Did your rebuild kit come with a ring kit? you stated that you didn't see one in kit.
most of the late model rebuild kits use transtec rings, later kits replace the steel rings on that servo with what they call lathe cut teflon ( plastic ) rings- will bet this is why you cant find ring- try fitting teflon rings left over in your kit will bet it is there.
Teflon will replace steel with no change in durability or sealing performance.
both the rings for the piston can be replaced with teflon, be shure to overlap ends correctly on install, lube with gell and will slide right in.
Lon;