I'm guessing that the access hole to the center chamber was designed so that it would be larger (wide open) when whatever contents it contained were put into it. And that the adjustable sleeve was designed so that when the contents were remove they could either be shaken or poured out in smaller, measurable incriments.
For example :
If it was designed to contain a "powder," the open position would allow it to easily be filled. And then when opened just "slightly" the powder could be shaken out. It doesn't appear to be water tight, but it may be. It just doesn't make sense (at least not to me) that the center chamber was designed for a piece of rolled papper or anything like that such as would be used for a dispatch.
And if I am even "close" to being right ... then what form of "powder" could it possibly have contained?
SBB