The ornamental design for a spur for a gunstock, as shown and described.
FIG. 1 is a left elevation of a spur for a gun stock.
FIG. 2 is a perspective view of the spur.
FIG. 3 is an alternate perspective view of the spur.
FIG. 4 is a top plan view of the spur.
FIG. 5 is a right elevation of the spur.
FIG. 6 is a bottom plan view of the spur.
FIG. 7 is a front elevation of the spur; and,
FIG. 8 is a rear elevation of the spur.
The broken lines in FIG. 1 depict an unclaimed stock structure. The broken lines in FIGS. 2, 3, 6, and 7 depict weight cutouts and interior side features that form no part of the claimed design.