Hello and welcome to the forum.
Most often a query can be answered from the accumulated records and books kept by forum members but in this case it seems no one has the detail you need. This is probably because the sponsor's mark is (relatively) modern.
Probably your best course now would be to contact the Assay Office direct by viewing
http://www.assayofficelondon.co.uk and using the Useful Contacts page for the Deputy Warden.
If you discover who registered the mark would you please let the forum know.
Mike
.