Johnson stopped installing rivets on sheaths in about 1971. You can find sheaths out there that were punched for rivets but they had been cancelled so no rivets were installed. You will just see a little dimple where a rivet was to have been installed. This is a way to date sheaths. I don't think your sheath is a war time sheath. Technically our involvement in V/N ended in 73 but the war wasn't over until April of 75. So it was possible for a mod. 18 like yours to have been carried by a Marine in April of 1975 while evacuating the Embassy but since yours is marked to a specific aviation outfit I don't think it was used during wartime.