on end piece 10 of pin 3 and with chamfer 23 for facilitating the guiding and bending of blade 17 during positioning of the assembly these chamfers serve a similar function on removal the chamfered edges of cam portions 24 of piece 21 facilitate engagement with the elongated elements 26 in the case of an assembly 5 whose elements constitute plugs the elements have a tapered end for easier introduction into a bore 27 of the end piece then into piece 33 and against piece 21 in an assembly 6 without elongated elements 26 the pin 4 may be placed in alignment with bores 27 the chamfers 11 and 12 formed on the end piece 14 have the same function as chamfers 11 and 23 of pin 3 as will be seen further on the second blade spring 18 is secured like spring 17 to piece 7 but in the edge portion thereof blade spring 18 has at its end a solid piece 28 also forming a locking catch as shown in fig1 and 2 in fig2 it can be seen that the anti blow up devices which have just been described are situated in the middle of the faces of the assembly they are completed by pins 29 also carried by the solepieces 2 for engagement in appropriate holes provided in the corners of pieces 7 these pins may also be provided with an anti blow up device each of the solid pieces 28 comprises chamfers comparable to those.