There are also a few repop verions of the rings out there. I believe the ones from HBC have the squared off stem holes while this one (below) is a very good copy of the original oval holes. These are made through D&R and while they do differ slightly from an original, they are very good quality and are very close in original design.
HIGH QUALITY REPOP RING FROM D&R - Serrated edges are the dead give away.