I went ahead and ordered the one from LMR as well, despite it not having the o ring. The shipping cost for the MM plug was $24 (cheapest option) and I couldn't find anything else on their site that I desperately need right now.In order to use the factory plug you'd have to drill and tap the end, where the rubber grommet is, and install a plug. A 3/8 pipe thread tap and 3/8 plug works well for that. It looks ugly though...and as cheap as the aftermarket plug is...its worth it not to have the hassle. I bough mine from LMR. It didn't come with a new o ring so I used my old one. It goes at the end of the threads by the top of the cap.