Abstract: A recorder pen having a tube or cartridge with a closed chamber and an integrally formed sleeve for supporting a linear fiber nib in writing position therein. The chamber contains a dye solution impregnated fibrous material in engaging relationship with an angled end face on the nib. An end plug is forced into the open end of the tube or cartridge for sealing the chamber and for limited compacting of the fibrous material against the angled end of the nib. Integrally formed with the cartridge are a plurality of mating channel-shaped mounting members adapted to grip the arm of a recording instrument. Web means are formed across the opening of selected mounting members which web means are partially pierced by the arm end to accommodate narrower width arms. Ridges and lugs project into the opening through the channel-shaped members and between the channel-shaped members so as to engage the flat surface of the arm to accommodate for arms of thinner dimensions.