I've never noticed this before and blindingly obvious when I think about it! To stop the strap rubbing the neck they loop the tie string through it to pull it down (shown in this Cuneo painting)
BTW, loads of lovely images at the National Archives web site, if you haven't seen it already:http://www.nationalarchives.gov.uk/theartofwar/
Yes, in Mk VIs there's even a special loop to put the string through. (Edit: in fact there's a loop on my MkVII as well) It's the "ready" position I think.
If you shorten the straps right down then it becomes the "alert" position. Basically the difference being the bag is a couple of inches higher up in the alert.
It's still done with modern, PLCE respirator bags, except it's a bootlace type thing, not string.
When I want your opinion - I'll tell you what it is!
