Abstract: The implement comprises a barrel formed with a slot, a retaining clip carried by said barrel and disposed on the outside thereof, and a clip extension which is integral with said clip and extends in said barrel and has a laterally bent lower end portion extending into said slot. Said extension is formed with a recessed ball seat. A pressure-operable mechanism comprises a slider slidably mounted in said barrel and formed with a heart-shaped groove, and a ball seated in said ball seat and protruding into said groove. To assemble the implement, the clip extension and slider, with the ball seated in said ball seat and protruding into said groove, are jointly inserted into said barrel until said laterally bent end portion enters said slot.