During the Second World War, Britain had designed eight variants of the Light Respirator from Mk. I to Mk. V. During the early post-war years, the transition from types fitted with the simplistic L2 valve holder began, in favour of those fitted with the L3 valve holder due to its inclusion of a speech transmitter, also known as a voice diaphragm. In 1951, a request for a new design of respirator was put forward to become the new service respirator, making the wartime models obsolete. This mask was the Mk. 6 Light Respirator. Serving alongside upgraded Mk. III and IIIAs from the war, the Mk. 6 became the general service respirator of the 1950s and continued service alongside one final model, the Mk. 7, into the 1960s. These masks saw service in the Korean War and the Aden Crisis, they saw service at the Chemical Defence Experimental Establishment as the "Porton-type", and later saw continued service with the Police, Fire & Rescue