Hi Frenchman,
I am an idiot. I have a lot of reference books and I forgot I had a copy of Ray Riling's powder flask book. He has photos of that flask but provides frustratingly little info about the manufacturer. Anyway, he associates the flask style with derringer-like small percussion pistols. Consequently, the bottom compartments typically were set up as follows:
1. The long compartment is for powder and the other is for balls, or
2. If it has an upper powder chamber separate from the lower ones, then the two lower compartments are for balls and caps.
dave