22 and a free end of the handle and the strut being pivoted adjacent to its other end on the lever 23 at a position between the axis 24 and a free end of the lever the handle 21 strut 25 and lever 23 together constitute a toggle linkage which provides a large mechanical advantage to the handle as the strut becomes aligned with the handle the clamping mechanism further comprises a pressure plate 26 disposed within the aperture 14 and guided for rectilinear movement relative to the fore end portion 10 along the length of the stock a guide pin 27 extends from the pressure plate 26 into the slot 13 on the end portion of the lever 23 remote from the axis 24 there is provided an adjustable abutment 28 which by pivoting of the handle 21 can be moved into the slot 13 and engaged with the guide pin 27 to urge the pressure plate in a direction away from the free end of the fore end portion 10 it will be seen that the clamping mechanism provides a considerable mechanical advantage to a user who grasps an end portion of the handle 21 remote the axis 22 so that a user can apply a relatively large force to the pressure plate when the handle 21 is pivoted to the position illustrated in fig3 the abutment 28.