carried by a rod 71 the end of rod 71 is pivotally connected at 72 to a small lever 73 having a surface 74 capable of sliding on the outside ledge 75 of a bushing 76 inserted in the hole 69 a spring 77 is inserted around rod 71 between bushing 69 and small piston 70 small piston 70 has a slot 78 into which penetrates a pin 79 perpendicularly to the axis of the small piston and integral with a latching block 80 that can slide inside a second hole 81 machined in support 78 block 80 is kept constantly pressed to the bottom by a spring 82 whose tension can be regulated by turning a screw 83 against which the spring 82 presses in support 68 there is a slot 84 in which pin 79 can travel in block 80 a seat 85 is machined which is capable of mating with wedge 86 that is secured by a screw 87 to a projecting part 88 of tubular element 29 secured at the bottom part of support 68 is a locator 89 in which is inserted an adjusting screw 90 coaxial with screw 87 and adjustable to the position desired by means of set screw 91 when spring 82 presses the seat 85 of block 80 against wedge 86 any rotation of tubular element 29 around axis 47 is prevented however the rotation of the.