General Motors was the first U.S auto manufacturer to mass produce the pillar-less hardtop body style. GM applied the moniker “Convertible Hardtop” to the 1949; Buick Roadmaster Riviera, Cadillac Coupe de Ville, Oldsmobile Ninety-Eight Holiday. The term “Convertible Hardtop” was derived from the concept that they would build a convertible and add a permanent hardtop that resembled a convertible with the top up. The doors would dispense with the fixed metal framing around the door window glass. This concept was first applied to two door cars and spread to four doors cars and some station wagons. The style became very popular and even struggling Independents had produced their own Hardtop models. Almost all U.S. Auto Makers had a Hardtop available by the mid-1950s.
The pillar-less hardtop was significant enough for most car brands to attach a corresponding name (in parenthesis in the image descriptions above) specifically for that body style or top tier model or trim level that was only available as a hardtop body. Models that were aimed at the economy conscious often did not offer hardtop variants. Conversely, upper market models would sometimes eliminate sedan versions from the line up.
Hardtop Brand Monikers:
Chevrolet –> Sport Coupe (2 door) Sport Sedan (4 door) confusing the issue since the term sedan was relegated mainly for traditional framed door glass cars.
Pontiac —-> Catalina
—-> de Ville & Seville
Chrysler (& 1955 Imperial)
—-> Country Club
Notice that some of the names would be used again, becoming separate models of their own. (i.e. Catalina, Riviera, Lancer, Newport) or trim packages (i.e. Holiday, Landau)
Visual example of Sedan vs. Hardtop
1956 Chevrolet 210 Two Door Sedan
1956 Chevrolet Bel Air Four Door Sedan
1956 Chevrolet Hardtops; Bel Air Sport Coupe & 210 Sport Sedan
Federal regulations that allowed for Quad Headlight systems went into effect in 1957. It did present a problem for U.S. Auto Manufacturers, because the four headlight systems were not legal in all states. Each company (sometimes each marque) handled the situation differently.
Chevrolet 1957 Bel Air & 1958 Impala
GM is the easiest, from Chevrolet to Cadillac all GM cars kept the Dual Headlights system for 1957. When all states legalized the Quad System, in 1958, all GM cars switched from two to four headlights.
1957 Cadillac Coupe de Ville
AMC (1957 the last year cars were marketed under Nash & Hudson names) The Nash Ambassador was sold with the vertically stacked Quad system. The Ambassador does not appear to be an option for states that had not legalized four, other than buying other AMC models that had Dual systems.
1957 Hudson Hornet Custom
Ford Motor Company handled it differently for different marques. Continental, Ford regular line and Thunderbird kept using Dual headlights. Most Mercury cars used two headlights, but the new flashy Turnpike Cruiser used an obviously adapted Quad system. Lincoln went another way. It looked like a vertically stacked Quad system, but in reality was the dual standard nine inch headlights. The lower lights were “Road Lights”.
1957 Ford Thunderbird
Chrysler Corporation also used a mix of tricks to handle the situation. The upper echelon marques: Imperial, Chrysler and DeSoto offered their cars with Dual or Quad systems. (Chrysler & DeSoto examples above) Dodge and Plymouth, on the other hand, used Dual headlights, but positioned and disguised parking lights to give the illusion of Quad headlights.
Studebaker/Packard was struggling and did not have the resources to handle dealing with the mixed regulations and would hold off adapting their designs for Quad headlights until 1958.
Depending on the response this post gets, a follow up on what happened in 1958 when four headlights became legal in all states may be assembled.