When I was trying to find a particular Swiss pack I ran across a small forum that had some informative posts...
So maybe you could find some help with military reenactors, military historian groups, forums. Some of the things I found were good pics of soldiers, pages from field manuals and stuff like that.
Might be a dead end but it would be easy to email some of the surplus sellers and see if they can get a manual that would detail intended use. I'd also ask for a used pack to use the material for your mods, add-ons