I've dabbled in blackpowder over the years mainly flintlock with some inline thrown in there. If it is actually blackpowder, the coarseness/fineness of the granulates would classically fall into either 'Cannon-F-FF-FFF-FFFF'. I couldn't locate a image that did a good job comparing them all side-by-side comparing I'll try to piece together a number of images that will hopefully illustrate the point...
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'Cannon'
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'F' & 'FFF' (kind of a potato photo but not much on the google)
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The GOEX lineup but as you can see not the best of details.
For a flintlock, I'd have to check my loads but I want to say I used something like 100-120 grains of 'FF' in the barrel and enough of the 'FFFF' to fill the flashgun. If it is indeed an alternative to blackpowder, I did find an image comparing some of the more modern alternatives that may be helpful to you identifying what you have.
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As you already know- powders are all different so aside from the curiosity aspect, I'd be awfully hesitant in guessing and putting in one of your firearms, both for your gun and yourself in case you guess wrong. Either way- hope that helps add some clues to go off of. Good luck on your journey figuring it out.
-LD