From Imperial War Museum website
Second World War period British Army issue gas mask. The Respirator, Anti-Gas, Light was issued from 1943 and first saw operational use with the assault troops during the invasion of Normandy in June 1944. More flexible than the earlier General Service Respirator, the filter screwed directly onto the mask, having no connecting hose. Initial issues catered only for those soldiers that could aim and fire their personal weapon at the right shoulder (the filter fitted the left side of the mask), therefore left-handed firers had to retain the earlier pattern masks until supplies of modified masks could become available. The Eyeshields were originally designed to counter the effects of airborne gas delivered as an irritant spray and the tin of Anti-Dimming compound was wiped around the inside of the respirator eyepieces to prevent moisture clouding the glass as the wearer exhaled. The Haversack carrier remained in service with some Territorial battalions until the mid-1970s.
They would only be used in combat if gas was suspected.