1851 UK Census Frances Douglass aged 39 years listed at 13 Saint James Street as a widow and working goldsmith and silversmith employing 8 men and a boy. Although the dates of her mark registrations overlap with Archibald Douglas's (Grimwade p.493) by a year at least, I suspect that "Frances Douglass" is the correct person for this Grimwade entry.
Archibald and Frances Douglas had a son named Archibald John Douglas who was born in 1834. He is listed in the 1851 UK Census with his mother as a silversmith.
DOUGLAS, Francis (Gimwade p.493)
Moderators: MCB, buckler, silverly
Re: DOUGLAS, Francis (Gimwade p.493)
An example of the work and mark of Frances Douglas, a lock-down inkwell, assayed at London in 1838:
F·D - London - 1838
F·D - London - 1838
Re: DOUGLAS, Francis (Gimwade p.493)
Two more examples of the mark (Grimwade No.677) of Frances Douglas:
F·D - London - 1838
F·D - London - 1838
F·D - London - 1838
F·D - London - 1838