center locking hand knob 14 and the right center locking hand knob 18 by sequentially turning each knob 14 18 in a counter clockwise direction this loosening action enables the rotation of the left vertical clamping ledge 11 and the right vertical clamping ledge 15 from their stored horizontal positions parallel to the slide rail 22 into vertical positions as shown in fig2 the height of both the left vertical clamping ledge 11 and the right vertical clamping ledge 15 are determined by the operator raising each vertical clamping ledge 11 15 within the confines of an open space provided for in a left machined slot 12 and a right machined slot 16 the left vertical clamping ledge 11 and the right vertical clamping ledge 15 must then be locked into their vertical positions by clockwise turning of both the left center locking hand knob 14 and the right center locking hand knob 18 it is also to be noted that a left flat clamping ledge 13 and a right fiat clamping ledge 17 are now positioned at the uppermost part of both the left vertical clamping ledge 11 and the right vertical clamping ledge 15 respectively in this manner the left flat clamping ledge 13 and the right flat clamping ledge 1 provide two points of support to the.