I was looking around through some of the 707 photos and I managed to find a photo of a TWA 707-331B which is a bit of a rarity. It is a JT-3D powered example with the abrieviated ventral fin (slightly shorter than the origional). FDXmech is correct about the yaw stability being the reason for it's installation. Most jets with highly swept wings suffer from a problem called "Dutch Roll". This occurs when a jet develops a yawing motion out of phase with a tendency to roll back and forth. It was difficult for the pilot to correct and led to some serious problems. You will also notice that all later model had(and most earlier ones modified with) an extension to the vertical stabilizer as well.
Here is a photo of the above mentioned TWA 707:
Click for large version
Photo © Ted Quackenbush