They are Hogue. They are only for the Centennial (and new polymer Bodyguard) because they cover the entire backstrap. (There's a plastic insert for the Bodyguard; it's not used on Centennials.) That cutout on the bottom is the polymer butt cap. In addition to helping that part of the butt avoid catching on cloth, it is also what holds the rubber onto the grip frame. It's an ingenious design, lightweight, and has no hardware.
I especially like them because they cover the entire backstrap--allowing a high hold--and provide just enough room for a pinky--they stick out 0.41" below the frame.