Roman copper-alloy gilt disc brooch with missing ?glass inlay

Roman bronze disc brooch, diameter 4.2cm. Decorated on front by a central cast cell, which evidently once held a stone now lost, and co-centric with it two cast ridges (one on rim itself). Flat zones between ridges have punched ornament of formal kind; mainly borders of SSS decoration. Front, except for central cell, has been gilded. Back tinned. Back has remains of a catchplate and broken pin attachment on raised ridge; iron stain around perforated lug suggest coil at least iron.