first plate 1 and is mounted on a set of slides 42 see fig7 and 8 allowing vertical movement in translation of the first shell support 6 relative to the first plate 1 this movement in translation is driven by a system comprising a screw 13 and a nut 14 this screw 13 has a head 15 bearing on a surface of the first shell support 6 so as to be able to raise the latter the head 15 being prevented from rotating by a lug 16 it will be noted that the lug 16 merely prevents rotation and does not impede movement of the shell support 6 in the vertical direction away from the head 15 and consequently from the first plate 1 the nut 14 is mounted on the first plate 1 in rolling bearings so as to be able to turn freely to cause the screw 13 to ascend or descend this nut 14 is driven in rotation by a gear system 17 with concurrent axes enabling the nut 14 to be rotated manually by a knob 18 a very accurate vertical position of the first shell support 6 relative to the first plate 1 is obtained in this way alternatively the movement in.