Dave I think it is proof that non bulk nitro powders were developed and used at a time when composite barrels were the norm and black steel in it's infancy, very late 1890's. If non bulk and bulk shotgun powder were being sold concurrently and they were, then I could see some blowups occuring from using the old tried and true powder dipper with non-bulk powder. Shell development changed again in the early 20's with the advent of the Super-X heavier loadings. So the real crux of the matter is that if your dam or twist Parker is in good sound condition and you load your shells to pressures and velocities for that time period you don't have anything to fear. One could in theory load paper cases using Unique powder with nitro cards and filler wads and have a pretty true to period shell.
Interestingly enough Unique is a double based powder (vs single based like SR7625 and SR4756).
"Unique (introduced 1900, still produced today), initially produced by Laflin & Rand, absorbed by Du Pont in 1902, given to Hercules in the 1914 anti-trust settlement. By Sharpe's words: one of the best of the entire Hercules line for mid-range use in the military rifle family or the so-called Sporting Rifle type such as 30/30, 3855 etc... one of the most useful and flexible powder a reloader can have available. A disc powder (.060" diam x .005" thick). RQ is 300. "
http://www.gmdr.com/lever/pistolpowi.htm