Granular size of the powder determines the intended use of the powder. Black powder granules come in sizes from the largest grain 1fg (fg) down to the very finest 4fg (ffffg).
The sizes are all suited for a particular type of use:
1fg: This powder has the largest grains and normally is not used in firearms, but if you happen to have a cannon, this just might be the powder for you.
2fg: The actual grain size of the powder is smaller than that of the 1fg powder. This powder is most commonly used in larger bored rifles starting at about .45 caliber and up.
3fg: This powder has an even finer grain and is used primarily in pistols and small bore rifles of up to .54 caliber.
Note there is some overlap between the uses of 2fg and 3fg. My personal preference, and I make no claims that it will work for everyone, is 3fg powder in pistols and all my rifles up to .50 cal, and use 2fg in all larger calibers.
4fg: This is the finest powder and is usually reserved for use in the flash pan on flintlock rifles.
I dont know when the grading as such came to be but this is how it is now......Probally not answering your question but this is all I could find.