hi Neil - can well imagine an author as renowned as Doyle having something more than just an ordinary ink well.
When the British Board of Trade commenced their system of Registrations back in 1842, they created no less than 13 separate CLASSES to accommodate the various materials which were being used for domestic and industrial designs, and metal was in fact CLASS I, so obviously considered an important material at the time - perhaps now it might be plastic (which they then didn't obviously have a CLASS for).
My feeling is that your glass inkwell design doesn't form part of this Registration, which is almost certainly for the metalwork only. There are, as we've seen previously on the Board, some designs which have been Registered as a shape to be produced in both glass (CLASS III) and ceramic (CLASS IV), and as a consequence carry a single Reg. No. for both CLASSES, but in those instance the protection is for a design, and other features, that are intended to be identical.
Will have a look at your No. when next I visit in a week or two's time, and post some pix for you to see - a very nice item.