Raimey, I'm not quite sure what you mean by "sourcing" T powder. There are official French proofmarks to indicate proof with the other smokeless varieties (J, S, M, and R) as well as T. However, according to Kennett, T was the last to be developed, in 1900. But Kennett adds that the others were all insufficient to meet the 12,000 psi minimum set by the Brussels convention and all (except T) were dropped in 1914. (That would be another good hint for dating a French gun: if the smokeless proof is anything but T, it had to be 1914 or earlier.) And T is the only one of those still being manufactured. Those powders were all the product of the French government, which maintained a monopoly on powder production in that country.
I'm also unclear where there were any American "rebels" in the 1870's.
