Map 20 of Maps of SEA in the 1960s

The barrier system concept was initially code-named “Practice Nine,” later became “Illinois City,” and then was called “Dye Marker.”  The overall concept was subdivided into two subsystems, a ground-based “... Strong Point Obstacle System (SPOS), popularized as 'McNamara's Wall' in the press, and a lesser known Air Supported Anti-Infiltration System.”  Dye Marker became associated only with the ground-based system deployed just below the DMZ.  On May 28, 1968 , the Dye Marker title for the SPOS was changed to “Duel Blade.”

 Plans called for the air-supported system to place sensors and munitions in the northwest tip of South Vietnam (where mountains prevented the clearing of land and the construction of the SPOS) and in the adjacent area of Laos (where political considerations would not permit a ground-based system).  The air-supported system was initially called “Muscle Shoals” and finally became “Igloo White.” ^[1]

 Igloo White was further subdivided into two subsystems, “Dump Truck” and “ Mud River .”  Dump Truck,the antipersonnel, sensor-based system adjacent to the DMZ, was targeted against the thousands of North Vietnamese troops who walked down the trails in Laos that bypassed the DMZ.  Mud River , the antitruck system, stretched west across the main roads covered by the FACs of the 23rd TASS.