<p>A connecting section (43g) of a binding part (43) shown in Fig. 33A has a latch section (43j) projecting from the center of the connecting section (43g) and also has lower guide sections (43i) on both sides of the latch section (43j). The lower guide sections (43i) are made to contact with the inner sides of upper guide sections (43m) of a connecting section (43h). The lower guide sections (43i) have tapered sections (43k) making contact with the upper guide sections (43m). Because of the presence of the tapered sections (43k), the area in which the lower guide sections (43i) of the connecting sections (43g) make contact with the upper guide sections (43m) of the connecting section (43h) is reduced to reduce friction between the connecting section (43g) and the connecting section (43h). The construction reduces a load acting on a circular arc section of the binding part (43) when the connecting sections are fitted to each other and improves the accuracy of the fitting.</p>
