a head plate containing a chamber;

an arm being a bent plate provided with two opposite ends, one end serving as a cam portion and having a slotted opening formed adjacent the cam portion, another end of the arm serving as an insertion portion, the insertion portion having a first plate and a second plate extending distally from the first plate at a different level therefrom;

a connector having a connection plate and an extension plate disposed at a right angle to each other, the connection plate having a recess formed on an inner side thereof with a first pair of spaced apart locking pieces disposed on opposing sides thereof for slidably receiving the first plate of the insertion portion of the arm therebetween and a second pair of spaced apart locking pieces disposed on the inner side of the connection plate for slidably receiving the second plate of the insertion portion of the arm therebetween, the extension plate having a hole being provided at a central portion thereof and a recess being disposed next to the hole, the extension plate having two sidewalls extending from an inner side thereof, each of the sidewalls having a protruding tab formed to project toward the inner side of the extension plate;

a locating plate having two lips formed at two sides thereof, each of the lips having a planar flange bent from a terminal portion thereof, the locating plate having a raised portion disposed between the two lips, the locating plate being inserted between the sidewalls on the inner side of the extension plate, the flanges of the lips being respectively slidably engaged between an inner surface of the extension plate and the protruding tabs, the raised portion being displaceably attached to the recess of the extension plate;

a pivot pin penetrating through the cam portion of the arm and the head plate to pivotally couple the arm to the head plate;

a spring tongue including a locating portion and a force-applying portion at opposing ends thereof with the locating portion being secured in the chamber of the head plate and the force-applying portion penetrating through the slotted opening of the arm and applying a bias force to the cam portion of the arm;

a first adjustment cam penetrating an opening in the connection plate between the locking pieces to displaceably link the connection plate to the insertion portion of the arm;

a second adjustment cam penetrating an opening formed in the recess of the extension plate to displaceably link the extension plate to the raised portion of the locating plate; and

a plurality of locating pieces and retainers disposed in the chamber to hold against the locating portion of the spring tongue, the locating pieces compressing the locating portion to secure the spring tongue in position, the retainers being disposed in correspondence to an edge of the locating portion of the spring tongue.