Apologies for not responding sooner, but I have been off work for a while. As Archivist of the Edinburgh Assay Office, I can confirm that the HMS (in a shield, and also HMS incuse) are the marks currently registered to Howard Stearn, a UK jewellery manufacturer.
The reason his mark is not on the Incorporation of Goldsmiths' Archive Database is because when I created the database I only included registrations up to 1986. This means a handful of important firms and individuals who are still in business since that date (and still registered with the Assay Office) are on the system. We also add (annually) customers whose registration with the Assay Office has lapsed.
Stearn registered in 1994 and is still a current customer of the Edinburgh Assay Office. His marks are noted on the 'Live' customer database of the Edinburgh Assay Office which is not accessible to the public.
I remember many hundreds of these silver crosses being hallmarked in time for the millennium. At the time I was working in the Hallmarking department as a refinisher, so if any of the crosses were badly hallmarked, I had to sand and polish out the marks so they could be marked again. I really grew to dread them!
(Archivist, The Incorporation of Goldsmiths of the City of Edinburgh)