Each Riser is drilled and tapped to accept accessories like sights and Berger Buttons. There are 4 riser sizes in the Matrix, a 48" (#5), one that makes 54" & 58" (#3), one that makes 62" and 64" (#2), and 1 that is used for 66", 68", and 70" (#1). If you wonder about the D and S- D stands for Right Hand (Destro in Italian), and S is for Left (Sinistra in Italian).