A little p.s. from me - having looked at Clive's diagram on this page showing the bathtubs, aren't the strengtheners the oval cups outboard of the main fixings on the page? with one pointed to by the words "Bottom machined skin panel"?
This looks like it's another layer of shear in order to fulfil the brief of working around the reported skin problems in that area. Just strange it had to break the surface like that?