I hope I can clairify the Signiture (Sig) rings,,,,they all use the plastic inserts,,,,the "Z" means that a particular set of rings fits a Weaver (Picatany--sp??) base/rail,,,,the beauty of these plastic inserts in the fact that they do not mar the scope tube when tightened ,,,and I have used an offset pair in one ring and a "0" in the other ,,,with no problems,,,,these inserts are self-centering (sorta like a trailer hitch ball) and no matter how much you "offset" (-5-10-20 thou.) they center in ring body and place no stress on the scope tube ,,,,,Roger