I have an 1882 H&H 500 BPE rifle that has this charge inscribed on the side of the action:
"charge 5 drams NA, case 3 inches"
Charge is way too large for it to be smokeless. Standard .500 3" BPE load was 136 grains black under a 340 or 440 grain bullet. "Charge 5 drams" equals 136 grains, so the reference is to black in a standard load.
I agree that "NA" has to be a reference to a brand of black, and likely a British one. I suppose "N" could suggest Nobel's, but I wasn't aware that Nobel's ever made black at Ardeer? IIRC, National Arms and Ammunition was in trouble by then, so it probably isn't them.
