If you go back into the old documentation, ie pre ww1, you will find references to the various granulations in terms of "Musket" "Rifle" "Pistol" . There are others, but I tried to determine what those meant, and how they correspond to the modern "F" designations. I believe the modern ideas of "2F in 40 caliber and above" et al, came about in the 1950s from early ML reinactors, and it somehow became entrenched as "Old Knowledge" But like so much BP info from that era, (Cleaning, fouling, lubricant, use of water) it's wrong.
I cannot prove it, but a theory of mine is that someone back in the 50s decided to call 4f "Priming" powder for his flintlock. This is apocryphal, as Flintlocks were primed with whatever powder was in the paper cartridge or powder horn of the user. Few if any Flintlock users, and certainly NO military, ever carried a separate priming powder. But this somehow got accepted as "Truth". Well then, what is this "Pistol" grade stuff? Well that must have been 3f. And so on. Then the johnny come lately cartridge guys accepted that the old frontstuffers must know what they're talking about . . .