base 3 so a they are held in a vertical position a front jaw 21 against which the slide 20 comes to bear is mounted facing the other end of the rail 4 as can be seen in fig6 and 7 the jaw is retained on the base 3 by a swivel joint 22 so that the jaw 21 can pivot in all directions in the space above the base 3 in vertical axial section as represented in fig7 the front jaw 21 has a c shape whose lower part 21 a bears against the slide 20 which when at rest holds the jaw 21 in a vertical position the upper part of the jaw 21 extends parallel to the base 3 and has an open v shaped notch 23 as can be seen in fig1 and 6 the lower part 21 a of the jaw 21 extends over only a part of the width of the jaw and over only a part of the width of the slide 20 which has a plane front face perpendicular to the longitudinal a of the base against which the part 21 a bears the springs 19 work in compression between the support piece 18 and the slide 20 and it is hence the same springs tat retain the lateral jaws 7 and 8 and the front jaw 21 the.