The British Army had started using Thermos style insulated containers for transporting hot rations to forward positions during World War Two. These cylinders had space for insulating material, usually cork, between the outer shell and the inner compartment holding the food. This insulation prevented the heat from food escaping and kept the contents hot for far longer than a standard metal container. Hot food is essential for troops in the field as it helps keep their body temperature up and is a far greater boost to morale than cold rations. Following the end of the Second World War the Army introduced a new thermos type container that was a similar diameter to its wartime counterpart, but taller allowing more rations to be carried in a single flask:The container is made of metal, painted green, with large white letters prominently stencilled around the bottom reminding troops ‘this container must not be placed on a stove or fire’:This is because, being a pressurised canister, if the flask is heated too much it would explode. Unlike earlier designs I believe the post war flasks used glass wool lagging rather than cork to insulate the contents. The lid of the canister is held on by three spring clips:The lid itself has three hooks for these clips to attach to and has a green outer ring made of metal and a black plastic inner disc:A moulded set of instructions explains that the central button needs to be depressed to release the vacuum inside the canister before the lid can be removed:The vacuum occurs because the soup, stew or tea placed inside the canister would be hot. Even with the insulation this will begin to cool and as it does the hot steam in the top portion of the flask would condense back into liquid. As this occurs there is less air pressure inside the flask and a vacuum seal is formed, much like what occurs in jam jars when hot jam cools. This vacuum would make it very hard to remove the lid, but by reintroducing air this seal is broken and the lid can be removed.
The underside of the lid has a large rubber gasket that helps keep the flask airtight:The interior of the flask is made of plated metal to allow it to be easily cleaned and kept hygienic:Removing the screws allows the interior to be removed in case any maintenance is required to the layer of lagging.
I am still trying to ascertain if there was any specific way of carrying this container in the field as it is heavy and awkward when it is empty so I can only imagine what it was like when full of a few gallons of food. There is no carrying handle on the top so, unless it was only transported by Land Rover, there does not seem to be an easy way to manoeuvre it across rough terrain.