Hi Clive,
Just musing with some perhaps groundless speculation. A quick cross reference between Heals and Grimwades throws up a few possible RM candidates, but one perhaps stands out a little more than the rest, Richard Morgan of Aldersgate Street.
Whereas the other possibles are recorded as Goldsmiths, Jewellers, etc., Morgan is described by Heal as a Plateworker, and thus it would seem likely that he was definitely a working silversmith, rather than just a retailer, and would likely have been an entrant in missing Smallworker's Register and thus missing from Grimwade.
We do know from Heal that Richard Morgan was working in 1766, and a search reveals that he was working in 1761 from him being recorded as a subscriber to
The American Negotiator:
As said, mere speculation, but if nothing else, the above may offer a path for further research.
Trev.