Thanks Frank. So my initial thoughts were it must have been for a lamp, with the back two holes for the wire to run through. However, I suppose the holes could have been there for a metal or other 'rod' to go through so with something attached at the top ? but then there is no hole for the rod (presuming rigid) to insert in the base of the glass. I remain convinced that this is part of a lamp for now and that the holes and groove in the outside of the base were for the wire to thread through or sit snugly against. I think it's quite early, but obviously not 1880's - so annoying the reg no is virtually invisible presumably from the satinising process.
m