it is hard to believe that you could be missing that piece and not have a leak out of the frame. but if by some magical weird way, you fixed the leak by changing an oring and not the gasket, i would just leave it. a sullinoid is expensive, but shopping around you could find just the gasket. maybe oringmonkey.com? anyway, try and fix the leak, and if it shoots without a gasket (i doubt it) then i'd just not use one.