From
Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, Volumes 1-2, 1897
'S.S.' was "Shotgun Smokeless" by the Smokeless Powder Co. (U.K.) which was eventually discarded as loads with more than 3 Dr. Eq. powder had substantially more pressure than those with "E.C." or "Schultze"