absence of a pin 61 results in the corresponding blade 33 not being advanced and thus remaining excluded assuming the desired number of pins 61 to be positioned as desired within the holes in rows 57 and 59 in a stand 39 and assuming the stand 39 to have been attached to its respective block 31 the pins 61 will instantly contact one of the stepped portions 33e or 33f however centripetal movement of the blades 33 contacted by the pins 61 may be prevented because the jacks are in a non raised position or are not raised to a position coinciding to the selection conditions required by the particular arrangement of the pins 61 in the stand 39 which is assumed to have been attached to its respective block 31 in order to preposition the jacks to be receptive to the desired placement of the pins 61 a cam 63 is mounted on a manually rotatably shaft 65 the cam 63 being adjustable by rotation of shaft 65 to act on a lower butt 13b of the jacks 13 causing all of the jacks 13 to climb in correspondence of the centripetally projecting blades 33 through engagement with their respective cammed surfaces 33a or 33b this prevents interference of the blades 33 with the butts 13c.