A sideplated boxlock (Kastenschloss mit blinden Seitenplatten). Boothroyd, "Sidelocks and Boxlocks", shows such a gun by Charles Hellis & Sons, Birmingham. Quote: "Such plates are sometimes encountered. They do not have anything to do with the mechanism but are there as decoration."
On this A&D by Westley-Richards they may have been added later to reinforce a split stock repair.