The ornamental design for a military service headstone medallion, as shown and described.
FIG. 1 is a top view of a military service headstone medallion showing my new design; and,
FIG. 2 is a top view thereof, the undisclosed rear is not part of the claimed design.
The broken lines in FIG. 2 are for illustrative purposes only and forms no part of the claimed design.
The insignia on the medallion forming a part of the claimed design is a registered trademark of the Department of the Navy, U.S. Marine Corps, 2 Navy Annex, Washington, D.C. 20380-1775.