US D621039 S1
The ornamental design for a grooved piston, as shown and described.
FIG. 1 is a perspective view of a top perspective view showing the new design for a grooved piston;
FIG. 2 is a side elevational view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
The portions of the grooved piston shown in broken lines form no part of the claimed design.