Abstract: A writing or marking implement comprising a cartridge unit; a housing for the unit; an operating mechanism including a pair of relatively rotatable driving members supported in axial alignment with the housing, and a follower slidingly engaging the driving members and reciprocating the unit between a concealed position and a point exposed position in response to relative rotation of the driving members; and a coupling providing a releasable threaded connection between the housing and mechanism whereby the mechanism and housing are selectively separable to permit removal or replacement of the unit, the coupling including interengaging wedged surfaces locking the threaded connection against inadvertent loosening under the rotary forces generated during repeated actuations of the mechanism.