SS was a marketing group started by Derby, kanotex, Vickers and a few other companies in the Midwest. The SS shield kind of held the station to higher standard(like AAA). Most stations would have both the Brand, like Derby, along with the SS shield signs at the station but I have seen a few pics of stations that only had SS signs. Hope this helps. It appears that you have the smaller of the two shield signs.
Always looking for more Derby, Vickers, KanOTex, & Elreco gasoline & oil items!