The base of the later finders, the part that holds the screen, is attached to the main body by 4 screws, two on each side. The location of these screws and the corresponding tapped holes could be the same as the original non-interchangeable model original to the D series. Or it could be that someone simply drilled and tapped new holes in order to attach this later part. In either case, it would have happened for just the reason you are pleased- a nice camera missing just one feature the owner wanted, and this was as good a way as any to get that feature.
JPD, do you think the serial number would have been engraved at the Rollei factory or an official repair center? Or would an independent machinist have done this? I don't know much about what kind of retrofitting services Rollei offered.