Pre 1900, probably. Powder S was in use for proof from 1896 until 1914. From what I was told by the proofmaster at St. Etienne, powder T came on in 1900 and was the standard proof powder, BUT, the customer could specify proof with powders J, S, M or R, after 1900 until 1914-but, few, very, very few, actually did.
You might want to keep your load pressure down with that old girl. The old powders were dropped from use as proof powders because they had trouble getting them to develop 12,000 PSI minimum pressure specified by the Brussels convention.
Powder T is the only one of them still in use today.
Best,
Ted
