Respirator, anti gas, light, Mk II and carrier
The initial design was brought forward in 1938 - but it took many years before it was finally introduced during WWII on the 27th of March 1943. It was initially meant for tank crews, paratroopers, commando units and other "specialised" units. In the end it was handed out to almost every unit in the British army, when available. It was easier to use, and made up smaller space than the General Service Respirator of the pre-war era.
Initially it was meant to be introduced in both a right and left hand version, but the army did not want to encourage left handed firing of the riffle - and the left hand version never went beyond the initial trials. Production costs may also have made the left handed version less desirable during the war. 
The rubber mask came in a small specially designed carrier, that contained the mask, anti-dimming agents, instructions and other anti-gas related items. It was not unheard of, that British soldiers disposed of the mask when in the field and used the handy little carrier for other items as the fear of gas attacks diminished as the war progressed.
The Mk I version of the mask was trialled in North Africa during 1942, and was found to be superior to almost any other mask in use with the allied forces. There was less than 1% serious leaks during a 695 man-run over an assault course. 
Each man would receive an extra red dog tag (of rubberised asbestos) that were attached to the gas mask - if the wearer were to be so unlucky to be killed during an chemical attack. 
My own mask is a late war version, with the new harness. It is stamped with the Danish Civil Defence organisation (Civilforsvaret) stamps - CF. In Danish service the mask was designated M/45 E. (1945 - English)
 Mayer-Maguire, Thomas & Baker, Brian, British Military Respirators and Anti-Gas Equipment of the Two World Wars, The Crowood Press Ltd., Ramsbury, 2015.