Here's my take on the stopper...it could be for a separate wine bottle. To cap it after it is opened but when you don't finish the whole bottle (rare). Although it looks like it fits this neck and compliments the set's finish, it may have been one that was added over the years because it fit. I think it is odd also that the original stopper would sit that high on that fancy pouring neck/lip. I would have expected the original stopper to follow the contour of the opening. The cherub on the stopper is similar to the cherubs on the metal work but not identical. That is why I think it was added later.
Oceans asked "What happens to cork after 50+ years?" Nothing, it just is cork. Cork has been used on fishing rod handles for well over 100 years and it is still cork. It might be worn and dirty but cleaned up it is good as new. It does not rot, crumble, or disintegrate. It might shrink a tad which is why wine should be stored so the cork stays wet and seals the neck as it was meant to.