other fastening means the protruding member 124 is sized to be received within a selected one of the grooves 214 to prevent axial movement of the clamping member 100 relative to the rail mounting member 210 e g due to recoil of the firearm when a round is fired the mounting base 110 includes a pair of arms 126 which are spaced apart in the axial direction and extend generally downward on the opposite transverse side of the mounting surface 114 as the second inner surface 122 the pressure plate 112 is slidably received between the arms 126 and is slidable in a direction orthogonal to a proximal inclined surface 212 b of the mounting member 210 the pressure plate 112 includes a first rail engaging surface 128 which engages the proximal inclined surface 212 b in aligned and facing contacting relation when the unit 100 is attached to a mounting member 210 the pressure plate 112 also includes a cam engaging surface 132 opposite the rail engaging surface 128 the pressure plate 112 includes a projection 132 having a pair of rails 134 defining a dovetail slot 136 a sliding locking tab 138 is slidably received within the dovetail slot 136 although the slot 136 and tab 138 are shown as having complimentary generally female and male dovetail shapes respectively it.