Hi Stu.
http://www.vikingsword.com/vb/showthread.php?t=17586
I posted a similar one two years ago this month and received some great responses. You might want to read.
We know the larger containers with the hinged lids that were fabricated from brass, silver, etc. were used to carry pre-made combustable cartridges, often used with the large Horse pistols.
But these small ones are somewhat of a curiosity. And, as Kuber mentions, many of these small ones are probably not as old as we would like them to be.

But every one I've seen are made as a casting. Could be used to carry loose lead balls, grease or pre-greased patchs/patch material, flints, percussion caps, or ? I don't think anyone has been able to confirm their primary use. I wish I knew.
Thanks for posting. Here is mine that I posted two years ago.
Rick.