The ornamental design for a bench back, as shown and described.
FIG. 1 is a perspective view of a bench back showing my new design.
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The features presented in broken lines, including the floral pattern and text, represent environmental structure and unclaimed portions of the bench back and form no part of the claimed design.