Used to have one of these. Two extra straps to attach to the parachute harness. Also usually worn wrongly, even by those who should know better. Large flap towards the chest and string through the strap, pulling it down the back (away from the neck).
The pack without the mask was sold widely in the immediate post war years from army surplus stores as 'Gas Mask Haversacks' worn with strap diagonal so that the pack hung on the side they were just right as substitutes for the leather and much more expensive school satchels - and you could stuff more in them. Usually Khaki coloured but RAF blue ones were quite common. Having a dark blue navy one provided a bit of kudos. Plumbers etc found them useful as tool bags.
