This is one of several recent additions to my collection, and something I have been after for years. Its a MkIV fitted with the additional EA filter in the centre of the hose. These were mainly used (or so I have been led to believe) by the British Expeditionary Force or BEF for short. As a result of the well known retreat/evacuation from Europe back to Britain many of these respirators were lost due to them being abandoned along with other non essential equipment, or captured soldiers being removed of them by their German captors. This is one of several pieces that I really wish had a story to go with it, but unfortunately like the majority of items of collectable militaria, we'll never actually know.
Interestingly this particular example features the early facepiece design with the different exhale valve and eyepieces, the only example of this type I've seen which does.