Federal law had long mandated that cars sold in the United States have round, sealed-beam headlamps. Originally only 7" diameter units were allowed, but beginning in 1957, smaller 5.75" lamps could be used. That's why quad-headlamp styling became popular in the late Fifties. In 1974, rectangular sealed beams were permitted.
In 1968, the Federal Motor Vehicle Safety Standards were changed and clear covers were no longer allowed in front of running headlights, which is why the 1972 Citroen SM and the 1969 Jaguar XK-E Fixed-Head Coupe below have weirdly exposed headlights, unlike their Euro originals.
The standards were set to be updated again in the mid Eighties so as to allow aerodynamic composite headlamps like the rest of the world, but the final approval didn't come down in time, and so the 1985½ Ford Mustang SVO had to be finalized and sent to production with a very unfortunate looking snout.
