vertical walls of the slot 6 are provided symmetrically with respect to seats 8 8 u2032 said seats two for each side are symmetrical with respect to both vertical planes of symmetry of the slot 6 this refinement in fact allows to fit the cartridge on the body of the cutter holder from either of the two insertion sides of the slot 6 and therefore obtain a rotation of the cutter through 180 u00b0 about the vertical axis the seats 8 8 u2032 have a horizontal base a vertical wall and an oblique upper wall so as to have the minimum distance with respect to the base in the innermost point of the seat once the cartridge has been inserted in the cutter holder body therefore the pins 7 abut against said oblique wall of the seats 8 fig2 shows that a lever 9 for actuating the locking mechanism is lifted in this position the lever 9 is in neutral in other words the cartridge 2 is inserted in the cutter holder body 1 but is not locked by turning downward the actuation lever 9 as can be seen in fig4 the cartridge is also locked on the body of the cutter holder fig3 and 5 clearly show the elements that compose the locking mechanism the actuation lever 9 not shown is interlocked in a seat 10 of a crank 11 at the opposite end of the crank 11 with respect to the seat 10 a first hinge 12 is provided which connects the crank 11 to a link 13 the crank 11 is rigidly coupled to the front and central portion of the dovetail slider 4 by means of a second.