The ornamental design for a bench back, as shown and described.
FIG. 1 is a perspective view of a bench back comprising my design and illustrating unclaimed elements (seat and frame);
FIG. 2 is a perspective view of a bench back comprising my design;
FIG. 3 is a front end view of a bench back comprising my design;
FIG. 4 is a top end view of a bench back comprising my design;
FIG. 5 is a back view of a bench back comprising my design; and,
FIG. 6 is a left end view of a bench back comprising my design.
The broken line showing of the bench seat and bench frame in FIG. 1 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.