FIG. 1 is a perspective view of a mouthpiece showing my design;
FIG. 2 is a further perspective view thereof;
FIG. 3 is a yet further perspective view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
Portions of the mouthpiece in FIGS. 1-6 depicted in broken lines do not form a part of the claimed design.