The hinge section (24) integral with the plate section (20) attached under door section (17) has thicker cross-section (20a) at its base end. Reinforcing rib (40) extend through the bent portions (34a,34b) of the hinge (34) upto the front end of plate section (20). The bent portions of hinge are comprised to extend on rupture of the door and then swing outward. Gaps are formed between extensions and outer edge of shooting aperture and between extension and longitudinal ends of hinge section. The hinge including the bent portions are integral with the back up section.
