I'm often less than comfortable with many of the attributions of plater's marks. Far too often in the past attributions have been made purely on the strength of initials fitting names or similarity of marks to one another etc. Once these appear in print, they are taken as gospel and never questioned, but as most of these marks were never registered or recorded at the time, there is often little evidence to back up such attributions and quite a few must be considered into the Possible/Probable way of referencing them.
This is why I wrote 'usually attributed' to this mark, as the more I think of it, the less comfortable I am. Roberts & Slater appear to disappear around 1860 when they became Roberts & Briggs, and the Queen Anne pattern I've always associated with the 1880's at the earliest. It's possible of course that the pattern was around earlier than I thought, but until something concrete turns up I would be a little wary of the attribution and keep an open mind on it..
Hopefully other members will add there thoughts.