The ornamental design for a mail box flag, as shown and described.
FIG. 1 is a perspective view of my new design for a "mail box flag", it being understood that a flag having my new design is supported and held by a mail box for rotating to an up position when in use, as illustrated;
FIG. 2 is a right side elevation view of the mail box flag of FIG. 1;
FIG. 3 is a side elevation view of the mail box flag of FIG. 1, the opposite side elevation view being essentially the same as the side shown; and,
FIG. 4 is a left side elevation view of the mail box flag of FIG. 1.
The broken line disclosure of a mailbox and post is for illustrating the environmental placement of the mail box flag of my new design and forms no part of the claimed design.