This pair of salts was made in 1733 and they are of superb quality. I'm not sure of the maker however; the visible letters are IS but there might be another letter above them or it might just be an irregular cartouche.
This is James Smith 1st (Grimwades 1643) have a look at the link photo, the S looks very similar and could the mark above be the crown or pellet ? (cant make out what it is)