Hello.
The letters might be WS&S.
The WS&S mark with a signpost trademark has been attributed to two firms, W. Shirtcliffe of Sheffield (1921-1931) and William Suckling, Birmingham (c.1895-c.1960).

William Suckling later used the signpost mark with his modern S.Ltd in an oval mark.
http://www.925-1000.com/silverplate_S.htmlNot sure what the extra mark is, but my guess is that your marks were used by William Suckling.
Pat.