I claim the ornamental design for a headstock portion of a musical instrument, as shown and described.
FIG. 1 is a top, front perspective view of a headstock portion of a musical instrument showing my new design;
FIG. 2 is a front view of the headstock portion of the musical instrument of FIG. 1;
FIG. 3 is a rear view of the headstock portion of the musical instrument of FIG. 1;
FIG. 4 is a top view of the headstock portion of the musical instrument of FIG. 1;
FIG. 5 is a left side view of the headstock portion of the musical instrument of FIG. 1, the right side view being the mirror image of the left side view;
FIG. 6 is a front view of the headstock portion of the musical instrument of FIG. 1 shown in an exemplary environment; and,
FIG. 7 is a front elevational view of the headstock portion of a musical instrument of FIG. 1.
The broken lines showing the remainder of the headstock and the musical instrument are for illustrative purposes only and form no part of the claimed design.