The Zwitter tank has this unique big and square top with the 60mm filler neck like original fit.,  Fits Beetles 1952-1955 and Karmann Ghia.  It has flat top and under it has square hump in the center like original,  it has fuel filler neck of 60mm this is what it takes for fuel cap.  The neck has inner locking teeth so you need fuel cap that fits this which our part number is 211201551.