Part of the Osprey Mk II and Mk III body armour set included a pouch holding a helmet bag. The helmet bag is designed to give a soldier a safe place to stow his helmet when it is not needed, somewhere it will be secure, out of the way but easy to access if it needs to be quickly donned. A lot of troops issued with the PLCE sets used a climbers carabineer usually attached to one of the shoulder straps or the waist which was used to clip the Mk 6 helmet to. The Osprey system gave troops a dedicated bag for this and although I do not have the pouch it is stowed in, I do have the mesh bag:The bag is designed to be large enough to fit a Mk 6 helmet in reasonably snuggly so that it doesn’t rattle around:The neck of the back has a drawstring and an adjustable strap allows it to be attached to the Osprey system:As well as the drawstring, two large Velcro panels are also provided to help secure the neck:I am not sure how popular this helmet bag was in service as I have struggled to find much information on it, one thing I did find referenced was its use as a dump pouch for used magazines. In combat there often isn’t time to carefully stow used magazines away, but equally a soldier does not want to just drop them on the ground where they might get damaged, lost or stolen by the enemy. A dump bag is just an open bag that used magazines can be dropped into until there is a lull in combat when they can be put back into pouches. This large open mesh bag would probably be well suited to this role and would obviously not be needed for its primary purpose when contact with the enemy had been made.
My thanks got to Michael Whittaker for kindly letting me have this piece.