Coach,
This was a design used in the 1950's. It allowed a tank storing a volatile product like gasoline,to take some internal pressure so the product would not vaporize off. Today tanks have internal and external floating roofs for this purpose. When a customer tells me to build what I wish, I like to construct some of these unusual tanks.
Tom,
If you are constructing things, I would like to see them on this thread. It's not about how precise it is, it is about constructing and having fun.