Development of the M16 30-round ammunition case began in January 1966 when Natick Labs received a request from U.S. Army Weapons Command for a pouch to carry the forthcoming large capacity magazines. The resultant nylon case could accommodate three magazines and had a riveted metal insert with opposing stainless steel leaf springs that exerted pressure on the magazines to prevent them from rattling or falling from the case. It also featured a plastic spring catch fastener that permitted one-hand opening and straps on both sides for attaching hand grenades.1
The pouch was attached to the belt via two slide keepers and to the metal loop on the suspenders by means of a support strap with snap hook. The 30-round cases were rarely issued in Vietnam due to the scarcity of the magazines.
1. Support To United States Army In Vietnam - Clothing & Equipment (United States Army Material Command Feb 1967)