This is a Wilson Combat Mag Well, model 188S.
The Wilson Combat Custom Magazine Well provides a large beveled opening for super fast magazine changes without adding unnecessary bulk to your pistol. These magazine wells are manufactured from solid steel to provide a durable and attractive addition to your firearm. The Custom Magazine Well attaches to the grip screw bushings and is held in place by the grip panels. No frame modification is required. Minor modification to the grip panel is required unless relieved grips are used.
Features:
Large Beveled Opening
Solid Steel Construction
Allows Faster Magazine Changes