The lace-in zipper was developed as part of Operation Safe Step, which began in 1968 with the aim of reducing the incidence of foot fungus disease suffered by troops operating in the Mekong Delta. Laced into standard Tropical Combat Boots, the zippers enabled soldiers to rapidly remove their footwear during rest periods to dry their feet. Regular drying was proven to greatly retard the progression of dermatological conditions.
During 1969 elements of the 9th Infantry Division in Vietnam successfully tested the zippers along with quick-drying nylon socks and comfort shoes. The lace-in zippers were consequently type classified as standard A in May 1970 and were produced in 3 sizes (8-eyelets, 9-eyelets, and 10-eyelets). Though the 2d Brigade of the 9th Infantry Division1 had left Vietnam in July 1969, the zippers were issued to infantry battalions and Special Forces, with priority given to those units operating in the southernmost areas.2
1. The 2d Brigade of the 9th Infnatry Division formed the Army component of the Mobile Riverine Force
2. E247, ENSURE, Records of the U.S. Army Concept Team Activity, Records of the United States Forces in Southeast Asia, 1950-1975, Record Group 472, National Archives at College Park, MD