Thanks for all your help and an interesting discussion. Mystery solved, and I’m relieved I don’t have a shrapnel shell/ grenade.
As far as I understand, provided black powder is kept dry and sealed from the air there’s no reason why it won’t work, even if its 300 years old. I don’t think it decomposes by itself.
Nice collections. Fernando, what is your fuse in the acrylic box made of? Is it like hemp match, the sort used for matchlock musket?