move upwardly and downwardly as shown in fig1 the inside of forward side wall portion 440 comprises horizontal flanges 76 and 78 disposed below and above vertical slot 48 to prevent arms 26 a and 426 a from moving upwardly and downwardly as shown in fig1 rear side wall portion 450 comprises round opening 56 arm 26 b comprises the round end having round opening 58 arm 426 b comprises a round end having a round opening 458 pin portion 84 of pivot pin 80 extends through opening 56 and openings 58 and 458 and is fastened with another threaded lock nut 74 thus arms 26 b and 426 b are coaxially movably mounted on rear side wall portion 450 like pivotal member 60 pivotal member 60 a may move vertically between a release position in which pin portion 64 a engages with the upper end of vertical slot 48 and a latch position in which pin portion 64 a engages with the lower end of vertical slot 48 when pivotal member 60 a is in the release position pin portions 64 a and 84 are transversally aligned and forming a pivot axis such that housings 20 and 420 can pivotally rotate from a close position to an open position when housings 20 and 420 are rotated to the open position the rotative movements are stopped because the upper edges of arms 26 a and 426 a are engaged with corresponding side edges of inwardly tilted extending portion 442 of side wall portion 440 respectively in this situation both slots 54 and 454 are completely superposed with slot 48 and permit pivotal member 60 a.