The ornamental design for a component part of writinginstrument, as shown and described.
FIG. 1 is a perspective view of the invention;
FIG. 2 is a side view of the invention;
FIG. 3 is a front view of the invention;
FIG. 4 is another side view of the invention, showing the side opposite that shown in FIG. 2;
FIG. 5 is a back view of the invention;
FIG. 6 is a top view of the invention; and,
FIG. 7 is a bottom view of the invention.
The portions shown in broken lines in FIGS. 1-7 are included for the purpose of illustrating features on the writing instrument that form no part of the claimed design.