configurations to cooperate with the configuration of the barrel 12 with which it is used in the embodiment shown the barrel section 16 has a necked down or frusto conical portion 28 that tapers toward the forward opening 26 and has a cylindrical neck 29 that defines a central opening that terminates in the forward opening 26 the central opening of the neck 29 closely receives the neck 24 of the nib 22 a shoulder 31 may be provided on the nib 22 to engage the neck 29 to prevent further passage of the nib 22 through the forward opening 26 the neck portion 24 of the nib 22 that extends into and is received in the barrel section 16 may penetrate into or otherwise be in contact with the filler material 20 to facilitate communication of solvent within the interior of the barrel 12 to a forward tip 30 of the nib 22 which projects out of the forward opening 26 of the barrel 12 in certain embodiments the nib 22 may essentially form a seal or cap the barrel with the opening through which the nib extends being the only opening of the barrel through which the interior of the barrel may be accessed such as filling the interior of the barrel with solvent during its manufacture this may allow the use of a unitary barrel without requiring a separate seal cap or barrel section as described previously the nib tip 30 may have various configurations one suitable configuration is that of a chisel tip such as those that are commonly used on markers and the like where the tip 30 is set at an angle relative to the longitudinal axis of the instrument 10.