I claim the ornamental design for a mail box flag, as shown and described.
FIG. 1 is a perspective view of a mail box flag, showing the new design in a lowered position;
FIG. 2 is a perspective view thereof, illustrated in a raised position;
FIG. 3 is a top plan view of FIG. 1;
FIG. 4 is a bottom plan view of FIG. 1.
FIG. 5 is a front elevational view of FIG. 1;
FIG. 6 is a left-side elevational view of FIG. 1;
FIG. 7 is a rear elevational view of FIG. 1; and,
FIG. 8 is a right-side elevational view of FIG. 1.
The broken lines showing a mailbox in FIGS. 1 and 2 and lines on screw fasteners in FIGS. 1-3 and 5 depict environmental matter and form no part of the claimed design.