The blank (100) has holes and slits (2,3) along its long sides, which form the ends of the housing when the blank is folded. There are circular holes (4) to accommodate a drive mechanism. Further holes (5) accommodate a trigger mechanism and there are holes (6) to accommodate a bearing plate. There are large circular holes (7) to allow adjustment of the mechanism and a slit (9) to accommodate a drive lever. The blank may be pre-folded to form ribs (12) and a narrow portion (14) in the region of a flap valve.
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