The "o-rings" keep the screws from working loose, a common problem. The grips on fitted with only one screw, which tends to work loose as the gun recoils in the hand. A lock washer from the local hardware store will do the same and easier to find. It's the pressure on the grips from tightening the screws that cause them to crack, also a common problem.
You can also purchase wood grips from Marschal's grip in Hungary. They start @ $65 shipped, fit well, and come in several different woods, styles, and finishes. He sends a set of lock washers with them. You can save the plastic grips for resell purposes.