I wrote about the original "hardtop" design from General Motors here and GM's second-generation hardtops here. As the first linked post mentions, I liked the first-generation GM hardtop design very much.
The present post deals with the first round of hardtop convertibles marketed by other American car makers in response to GM's successful venture.
I include this first-generation GM hardtop as reference for the designs shown below.
Chrysler actually beat General Motors by introducing a hardtop convertible in its 1946 Town and Country line. It lacked a panoramic backlight window and very few were built, so I credit GM with introducing the design concept to mass production. Chrysler's first serious hardtops appeared for the 1950 model year (the corporate line had been redesigned for 1949) and appeared on 1950 Dodges and DeSotos as well as Chryslers. The design was very similar to GM's, but wasn't as effective due to the blocky nature of the basic Chrysler Corporation body produced 1949-52.
A photo of a rather beat-up Dodge hardtop. The Chrysler shown in the previous image was lowered and stretched by an illustrator to make the car seem more attractive than it actually was.
Chrysler's Plymouth brand didn't get a hardtop convertible until 1951. This photo that I found on the Internet features yet another well-used car, but I include it because it shows that Plymouth hardtops got rounded lower corners on their backlights, unlike the other Chrysler Corporation makes.
Redesigned Fords were coming for 1952, but a hardtop was introduced for 1951. Its design is identical or nearly so to that of the tops on restyled 1952 Ford hardtops as well as 1952 Mercurys. It's not as graceful as the 1949 Buick's design, but at least does a presentable job of including a good deal of glass area, the goal of General Motors stylists in their 1950 second-generation hardtop designs for Buick, Cadillac and Oldsmobile (see the second link above).
Post- World War 2 Hudsons (introduced for 1948) had a long, semi-fastback roofline. This convertible-based hardtop is more in the bustle-back spirit. The design of the windows is awkward, unfortunately. A dog-leg style C-pillar and backlight combination such as Nash used (see below) might have worked better. But the real problem was that the convertible-dictated aft edge of the top resulted in the top being too stubby to accommodate a graceful profile and a satisfactory window treatment.
Packards were completely redesigned for 1951 and hardtops were included in the product mix. The design shown here is similar the the GM style, the main difference being that profile of the upper edge of the rear window is slightly curved rather than straight. Nevertheless, it looks good.
The entire Ford Motor Company product line was restyled for 1952. Hardtop convertibles were given greenhouse designs that were very similar to the one previewed on 1951 Fords (see above).
Studebaker's initial hardtop design was produced only for the 1952 model year, being replaced for 1953 by the classic Loewy Starliners. Given the high degree of talent in Loewy's Studebaker styling shop, it's little wonder that the 1952 Starliner greenhouse looks good.
Nash added hardtop convertibles for the 1952 model year. 1949-1951 Nashes had a bulbous, aerodynamic design that did not lend itself to either convertibles or hardtops. The restyled full-size Nash line did not include convertibles, but a hardtop was introduced. Nash Ramblers first appeared for 1950 and did have a convertible model (but with fixed side window framing), but no hardtop till 1952. Nash's hardtops featured a dog-leg C-pillar arrangement similar to that used on 1953-54 Chevrolets and Pontiacs (see second link above). This is a sensible hardtop convertible window style, though the proportions of 1952 Nashes and Ramblers were not helpful to the greenhouse designs shown here.
Willys returned to automobile manufacture for the 1952 model year and added a hardtop line for 1953. There were no convertibles. The hardtop design was pleasant, though not striking, and aside from the lack of a B-pillar, is the same as that of other two-door Willys'.