FIG. 1 is a front-right-top perspective view of a seatback of a bench, showing my new design; FIG. 2 is a front elevational view thereof; FIG. 3 is a rear elevational view thereof; FIG. 4 is a left elevational view thereof; FIG. 5 is a right elevational view thereof; FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof.