The relevant U.S. FMVSS 108 regulation is extremely prescriptive. An example is that the only legal headlamps on new cars from 1940 to 1983, were glass sealed beam type:

In 1940, a consortium of state motor vehicle administrators standardized upon a system of two seven-inch diameter vacuum sealed beam headlamps. These sealed beam units offered better brightness and aftermarket manufacturers offered conversion kits for earlier cars. The '39 Plymouth I owned as a teenager had such a kit installed.

The two-headlight sealed beam configuration was only system allowed in the U.S. until 1957. At that time, the four-headlamp system debuted on production cars (one high/low and one high-beam). Each glass lens was 5.75 inches in diameter. The system debuted on the '57 Cadillac Eldorado Brougham. Chrysler and Nash offered them on some models in states that permitted the new system. Most American cars had quads by 1958.

In 1967, glass covered sealed beam headlights, such as those on the Volkswagen Beetle, certain Chrysler models and the Jaguar XKE were outlawed, necessitating some front-end restyling to accommodate the new regulations.

In 1983, regulations were changed to granting a 1981 petition from Ford Motor Company, the 44-year-old U.S. headlamp regulations were amended to allow replaceable-bulb, nonstandard-shape, architectural headlights with aerodynamic lenses that could for the first time be plastic.

The new regs paved the way the introduction of first U.S.-market car since 1939 with replaceable bulb headlamps – the 1984 Lincoln Mark VII. It was the first American car to have flush, aerodynamic lights. Amost all of today's cars have such flush mounted headlights but in 1984 this was quite unique and gave the Mark VII a distinctive "face."

These shaped American headights were sometimes referred to as "Euro" headlamps, since aerodynamic front lights had been common in Europe for some time.

And actually that narrative skips over year that rectangular sealed-beams were first made legal, with a similar furor.