As you point out Jackson (p721) lists Salter’s “earliest mention” in the Cork records as 1821. However, a few pages earlier (p718) Salter’s mark is shown with that of Richard Garde as 1809-1830. We have to conclude that the records are not comprehensive and that Salter may well have been working and sending silver to Dublin for assay and sale in 1803. Alternatively “JS” may not be James Salter, but rather an unrecorded maker.
For anybody reading this who wonders how Salter got on in New Zealand I have to include the full text of Jackson’s footnote:
“James Salter with his family emigrated for New Zealand in 1840 in the “Sophia Pate” which was wrecked on the voyage, and all save one boy were lost.”