rectangle and has a substantially arc shaped u shaped cross section with a rising portion thereof along its longitudinal direction the plate 32 as shown has two ribs that extend along the longitudinal direction in the middle thereof the plate 32 has two substantially opposite longitudinal edges 40 and substantially opposite transverse ends a bent bottom edge 44 is formed each along both of the longitudinal edges 40 fig2 the opposite longitudinal edges are therefore directed toward each other the plate 32 can be configured in profile as an upside down u201c u u201d as shown the elongated plate 32 is made of metal or any other suitable material which has enough rigidity and can provide a stable attachment for other parts of the mechanism while light weight material saving and cost saving two holes 46 are provided in the plate 32 fig5 for receiving and attaching a bushing 48 respectively so that the mechanism can be fastened in a file folder of a notebook six additional holes 52 are positioned along the longitudinal edges 40 for receiving a ring that passes therethrough respectively those plates or shells that have other kinds of shapes including irregular shapes or those mechanisms that are formed in a file folder or a notebook integrally all fall within the scope of the.