The ornamental design for a string instrument headstock, as shown and described.
FIG. 1 is a front upper-left isometric view of a string instrument headstock showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a front upper-left isometric view of a second embodiment thereof;
FIG. 5 is a front view of a second embodiment; and,
FIG. 6 is a rear view of a second embodiment.
The wood grained pattern in FIG. 1 and the neck, string-retaining nut and holes for tuning devices that are shown in broken lines, are for illustrative purposes only, and form no part of the claimed design.