Any idea what the little vertical strap in the middle at the bottom (literally!) is for?
Z
It is for your coat (feltkappe).
The ruck is an old assault pack (stridssekk). Usually it contained the mess kit, extra ammo, two tent poles, 3 tent pegs, 5 meter rope, a little food and your sweater.
The diamond shaped tent "half" was rolled together and fastened over the pack.
When the field coat was issued (before my time) it was fastened on the bottom of the pack.
The tent half is not rolled to standard, but will do as an illustration.