The impressed Salopian “diamond” mark can also occur with an impressed SALOPIAN mark, although the two marks were usually used separately. Used on higher quality pieces.
According to the Encyclopaedia of British Pottery and Porcelain Marks (Geoffrey Godden, revised edition 1991) the “diamond” mark "was registered in 1882”. It has not been possible to independently verify this. The mark seems to have been used until c.1910.