Gene,
Prior to PT being designated the only proof powder, one could specify which powder they wanted a gun proofed with. For a few years after 1900, people were asking for proof with powder S, as it could develop higher pressure at proof than powder T.
I think it was the elusive “Wild Cattle” that found this little bit of trivia for us, but, I would guess it has been years.

I haven’t seen much of him lately.

Best,
Ted