The ornamental design for a bench back, as shown and described.
FIG. 1 is a front view of a bench back comprising our design;
FIG. 2 is a rear view of a bench back comprising our design;
FIG. 3 is a top view of a bench back comprising our design;
FIG. 4 is a bottom view of a bench back comprising our design;
FIG. 5 is a side view of a bench back comprising our design;
FIG. 6 is a side view of a bench back comprising our design;
FIG. 7 is a downward facing front perspective view of a bench back comprising our design.
FIG. 8 is an upward facing rear perspective view of a bench back comprising our design; and,
FIG. 9 is a front perspective view of a bench back comprising our design.
The broken line showing of the bench seat and bench supports in FIG. 9 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.