I'm of no help, but this might be of interest
The first mention of Greener SBTs was in Nov. 8, 1895 Sporting Life
"E. D. Fulford is now using a single-barrel gun, and claims that he can do better work with it as inanimate targets than, when using a double-barrel gun. The new gun is a Greener, single-barrel, 12-bore, weighing 7 1/2 pounds, with a 32-inch barrel, and can use from 40 to 60 grains of powder without any variation in pattern. The breech is extra heavy, and the gun throughout made to stand heavy loads."
60 grains of "E.C." or "Schultze" would be 4 1/4 Dr.Eq.!!
I don't know the importer/dealer
The next year Fulford was using a Francotte SBT imported by Von Lengerke & Detmold