PURPOSE: To make the cut edge of a film straight and prevent a failure in positioning the mounting location by a method wherein a virtual line that connects the points of sawtooth-shaped teeth which protrude outside along the edge of a cover flap is nearly horizontally straight except for the middle part where the teeth have a height not more than two times the height of the teeth at the other parts.

CONSTITUTION: Sawtooth-shaped teeth 7 protrude outside along the front edge of a cover flap 5. A virtual line X-X which connects the points of the teeth 7 is almost horizontally straight except for the lengthwise middle part where the height of the teeth is a little higher than that at the other parts. The higher the height of the teeth arranged in the middle part, the more advantageous in a middle part cutting method, however, that results in lack of the durability of the teeth and involves apparent danger and such a default that the line of the cut edge of a film F is not straight. Therefore, the height of line teeth at the middle part is designed within a range from one to two times the height of the teeth at the other parts (in case of one time, the pitch of the teeth is increased). The protrusion measure of the teeth is defined as a distance between the virtual line X-X and the points of the teeth at the middle part.