I think you're absolutely right Tom.
Grimwade says that Robert Sharp's mark has been cut down from the combined mark of Daniel Smith and Robert Sharp, and so you can still see the bottom of the DS. This is the case with the tureen's mark, and is very clear above the S.
I've included a picture of the partnership's mark which itself was cut down from a Richard Carter, D. Smith and R. Sharp mark.
Clearly it must have been expensive to have a new stamp made; much cheaper to file it down.
Miles
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