PURPOSE: To improve the productivity of a product, and to reduce the manufacturing cost by engraving a marking in a strip-like metal, folding a long tail, punching out a slot, and forming a bridge in a lower end part.

CONSTITUTION: A casing blank 1 is prepared by a strip-like or sheet-like metal, and a straight track 2 is engraved to easily provide a hair-pin bend on a long tail 3. Two markings 7 to fit a spring trigger projecting parts are engraved in the blank 1. A slot 8 and the blank 1 are formed by one or two or more shearing, and compression as necessary. The blank 1 is then folded along its axis of symmetry to form an end part 10, and the long tail 3 is folded to form a projecting part 9 in a hook shape. Because the welding can be dispensed with, the productivity is improved, and the manufacturing cost is reduced.