"Sur la pont" is simply a corruption of "Sous la pont" which was indeed the original. It referred to a dance venue under the bridge. It is one of these errors that quickly spreads and drives out the original - A bit like singing "for the sake of auld lang syne" instead of "for auld lang syne" which is the original. It has to be said that mistaking "Sous" and "Sur" in an easy enough mistake.