The design of the M69 vest was improved in 1968 with the insertion of polyethylene stiffeners into the fifth layer of ballistic nylon, which alleviated the tendency of the filler to become bunched.1 Initially the improved vest was issued with the traditional zip closure, but it was later upgraded to a Velcro fastener.
1. Personnel Armor Handbook by LT R. A. Green, J. A. Parish (U.S. Naval Weapon Laboratory June 1971)