element 11 against the butt 6 not being sufficient to cause the subsequent interception of the incline 55a against the lower part of the butt 8 thanks to the particular conformation of the cam 12 the said butt is not subjected to breakage the front surface of the butt 8 is in fact intercepted in the situation to which reference has just been made position k1 in fig7 by the sloping section 57 and this results in a consequent gradual elastic flexure of the tailpiece 35 the said deflection being made possible by the suddenly reduced area 35a because the butt is intercepted by the track c1 of the block 56 position k2 in fig7 the deflection continues the curved section of the part 53a of the successive cam 12 then has the task of re aligning the blade element 5 in the neutral position r when use made of the blade element 5 depicted in fig1 a instead of undergoing flexure the tailpiece 35 oscillates with respect to the axis thereof in contrast with the corresponding spring 60 the cams 12 13 14 and 15 the selector elements 11 are symmetrical with respect to corresponding vertical planes and thus the same considerations as above apply also to the movement in the direction t2 of the carriage to conclude the particular conformation of the blade element 5 and of the cam 12 depicted in fig6 and 7 enables the under mentioned advantages to be obtained the blade element 5 is not subjected to any stress in the event of the non selection thereof the blade element 5 is not subjected to any stress when selected correctly in fact the cam 12 attends to raising the element.