Museum car is dirty and potentially under sealed.
If you cleaned it up it would look the same
Museum car is original and looks the same as it it was made (because the engine was broken the car has not been used). Take a look at pictures from Fusi or any other book with a period photos. Cars have never been shiny underneath, the parts were rusty, unpainted, etc. The car from tubut post looks like Easter egg, it is perfect it looks like just made but not preserved. I understand it as American way of restoration (to make cars better than they were new). European school is to keep original parts, keep patina and preserve the originality. Take a look at cars from Lo Presto collection, they restored in a different way.