We claim the ornamental design for a seat back for an exercise device, as shown and described.
FIG. 1 is a front, side perspective view of a seat back shown on an exercise device;
FIG. 2 is a front, side perspective view of the seat back for an exercise device of FIG. 1;
FIG. 3 is a front view of the seat back of FIG. 1;
FIG. 4 is a rear view of the seat back of FIG. 1;
FIG. 5 is a side view of the seat back of FIG. 1.
FIG. 6 is a top view of the seat back of FIG. 1; and,
FIG. 7 is a bottom view of the seat back of FIG. 1.
The ornamental design that is claimed is shown in solid lines in the drawings: The broken (dashed) lines shown in the drawings are included for the purpose of illustrating environmental elements only and form no part of the claimed design.