Thanks!
I should probably say that the fit of the stopper and the inside neck is absolutely perfect, put it in any way you like, there is not the tiniest hint of a wobble. Both surfaces are polished completely smooth and shiney, when I put my finger in the neck, there is a ridgey bit where the bottom of the stopper sits, where the glass widens into the rest of the neck.
The bulb of the stopper is hollow - it contains a perfect tear-drop shape bubble, but the plug is solid - you can make all that out in the last image.