The ornamental design for the seatback cover, as shown and described.
FIG. 1 is a front perspective view of a seatback cover secured to an office chair;
FIG. 2 is a front perspective view of the seatback cover of FIG. 1;
FIG. 3 is a front view of the seatback cover of FIGS. 1 and 2;
FIG. 4 is a rear view of the seatback cover of FIGS. 1 and 2;
FIG. 5 is a left side view of the seatback cover of FIGS. 1 and 2;
FIG. 6 is a right side view of the seatback cover of FIGS. 1 and 2;
FIG. 7 is a top view of the seatback cover of FIGS. 1 and 2; and,
FIG. 8 is a bottom view of the seatback cover of FIGS. 1 and 2.
The broken lines in the drawings are for the purpose of illustrating portions of the seatback cover and the environment and form no part of the claimed design.