The G2 has never come stock with a pyrex lens. 1HI4X, I suspect you believe your wife's G2 to have a pyrex lens due to the notched retaining ring, but this is not the case, unless of course you swapped it out yourself. I'm not sure why SureFire decided not to use the snap-fit design used for the aluminium bezels with the nitrolon bezels for lexan lenses, but that's just the way it always has been.
Kumar, the notched retaining ring in the bezel of the G2 is usually secured there with some loctite to prevent it from coming loose on it's own. You'll need some sort of tool, such as a pair of needle-nose pliers or watch case opening tool to unscrew it counter-clockwise from the front. The retaining ring usually comes out pretty easily but if it doesn't, you might have to heat it up a bit in boiling water to loosen the loctite.
Once you've removed the retaining ring and the lens, simply replace the parts in the reverse order, replacing the original lexan lens with the UCL. If the UCL is only coated on one side, make sure the coated side is on the inside. Otherwise it doesn't matter which way you put it in.
By the way the black rubber gasket goes in first, followed by the lens, then the retaining ring. You can either re-seal the retaining ring with some epoxy or glue, or leave it unsealed if you forsee yourself removing it again for any reason (e.g. cracked lens).
