I believe that "100% cotton" comes from the provenance of the cellulose in the powder making process. Some nitro powders such as S were made using wood cellulose.
My comment on the powder word was intended as a friendly provocation, not to be taken too literally.
As for the PT, PJ, PS, PM actual meaning the only proof will be in the text of the official rule that defined it. That's gonna be hard to find...
I have zero proof of this, it's just a hunch based on my general knowledge of the French bureaucracy, and a number of relevant readings from that time frame.

Quant a General Journee (bless his soul), he might be just as full of s**t as the Lyon Armurier.

Best regards,
WC-