Abstract: The release control lever 14 is mounted on a shaft 15 passing through the casing 1 of the apparatus and which is rigid with a crank-shaft 19. The crank-shaft 19 is connected through a rod 20 to the middle portion 21 of a rocking lever 17 pivoted at one end to the control links of clamp 5 and at the other end to a rod 18 pivoted in turn to one end of another rocking lever 16 having its other end pivoted to the control links of clamp 4 while its middle portion 22 is connected to a fixed pin 23. The rotation of the release control lever 14 results in a translation of rocking lever 17 and a rotation of rocking lever 16.