tongue 246 slidably received in the frame 254 as shown and as will be further described below the pin 248 projects through a slot 256 in the frame and the tongue is permitted to move in the frame between a first lower position with the pin 248 located at a lower end of the slot 256 and a second upper position with the pin 248 located near an upper end of the slot 256 as illustrated in fig1 and 3 the clamping apparatus includes a forked retaining member 258 having a pair of lower tines 260 and an ear 262 extending from the lower end of each tine 260 the retaining member 258 is secured to the upper surface of the extendable arm 110 by a pair of screws 264 extending through slots 266 in the ears 262 such that adjustment of the retaining member 258 relative the longitudinal direction of the arm 110 can be made through movement of the screws 264 in the slots 266 prior to securement of the retaining member 258 an arched locking member 268 has its rearward end 270 pivotally mounted between the tines 260 adjacent a lower end of the forked member 258 by suitable means such as a pin 272 extending through the tines 260 and locking member 268 as shown an air cylinder 274 is also provided for actuating the locking member 268 and.