Yep that's cordite.. slow burning smoky stuff that was in the hold of the Lusitania when she was torpedoed.( Its also called "gun cotton" in some circles bcuase its made in strands I think) Lots of old "african" cartidges were loaded woth Black Powder or cordite and are therefore NOT restricted to the 50+ caliber rule (thanks to someone here for that info).
The 416 rigby was loaded with a BUNCH of those strands to get its power (old Shooter's bible article)
Didn't think there was much of it left around. Burn it or soak it in oil to render it useless. Burn it in the open.. keep your face away from it and don't even think of putting it into a closed container.