Carl, Look up Federal Motor Vehicle Safety Standard 108. "In 1968 when Federal auto equipment and safety regulations were initiated, the requirement for two large or four small round sealed beams was codified, thus freezing headlamp design for many years. At the same time, the new regulations prohibited any decorative or protective element in front of the headlamps whenever the headlamps are switched on. Glass-covered headlamps, used on e.g. the Jaguar E-Type, pre-1968 VW Beetle, 1965 Chrysler and Imperial models, Porsche 356, Citroën DS and Ferrari Daytona were no longer permitted and vehicles had to be imported with uncovered headlamps for the US market. This change meant that vehicles designed for good aerodynamic performance could not achieve it for the US market." The standard was revised in 1970 to accomodate a petition by Ford Motor Company to use square shaped sealed beam headlamps and has been revised periodically since.