Thank you very much Phil and Trev.
This sun face mark is the trade mark then. My first thought of S mark was "Sheffield" and second thought it might be an indication of solid silver with content lower than sterling. I do have some very identical pieces, as per photos, clearly marked "A" for silver plating but my questioned one has only this S mark for silver status, as Trev said or no indication at all if the S is for Sheffield.
As said I got a big lot of them, checked and observed that the sun face mark has eyes nose and mouth in S marked pieces but only has eyes in A marked pieces. Is this typical in Mappin ?
And as per photo, the S is in frame of a shield while A is in square frame.