Ive seen lots of decanters with glass stoppers both old and new that are hollow. What is the purpose of this. Does anyone know. First thought might be that it is for measuring out portions, but that can't be. As soon as you turn the bottle back rightside up it all pours back out again! If it is just for looks you usually don't hold the bottle upsidedown!
I asked John and he (of course) basically gave me a half hour answer and he still didn't know