Looking again at the other Stolzle three piece sets + tray my idea that the stopper might be from a missing flask now looks wrong. The other sets have a flask with a stopper, a trinket(?) pot, plus either an atomiser or a flask with what looks like a small flat finial?
As my set has the atomiser and the flask with stopper, the missing piece should be a trinket pot with lid, not another flask with a stopper? I think it then follows that the stopper has nothing to do with the set?
I suppose the only way to know what sort of finial the flask should have is to find a pot with its lid?