a casing;

a movable plate pivotally mounted to said casing;

a combination lock mounted in said casing, said combination lock having a rotary knob, said rotary knob having a shaft, two notched rotating discs sleeved onto the shaft of said rotary knob, said notched rotating discs each having a peripheral notch, two gearwheels sleeved onto said shaft of said rotary knob and meshed with said notched rotating discs, a spring member sleeved onto said shaft of said rotary knob, one of said two gearwheels contacting against said spring member, and a combination-alteration control device rotatably mounted to said shaft of said rotary knob and stopped against the other of said two gearwheels; and

a sliding member movably mounted to said casing, said sliding member engaging the peripheral notches of said notched rotating discs when said combination lock is unlocked, said sliding member being moved away from said peripheral notches of said notched rotating discs to push against said movable plate when said combination lock is locked.