The 20 pounder gun was used until the Royal Ordnance Factories introduced the 105 mm L7 gun in 1959. All later variants of the Centurion, from Mark 5/2 on, used the L7. Design work for the Mk 7 was completed in 1953, with production beginning soon afterwards. It is one of the longest-serving designs of all time, serving as a battle tank for the British and Australian armies from the Korean War (1950–1953) to the Vietnam War (1961–1972), and as an AVRE during the Gulf War in January–February 1991.