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The style of these marks are typical of British plater's marks, they are in a majority of cases, meaningless, and are just meant to imitate official British hallmarks.
However, in this case, there may be some clues to the maker. A shamrock and a harp contained in the marks likely suggests Irish origin. This coupled with the J·S initials points towards the possibility that these may be the marks of John Smyth of Dublin, a leading member of the trade in Ireland in the second half of the 19th century.
At this point, I'm only offering the name as a possibility and a route for further research.
The engraving on the side would be the family crest of a former owner of the teapot.