During the late 1930s and early 1940s, the general mind-set of American stylists was that future cars would feature rounded bodies whose shapes paid more than a little heed to aerodynamics. This can be seen in sketches and, in the early '40s, some clay styling models.
Some designs in that spirit actually reached production after World War 2. But the teardrop-shaped future proved to be a false one because General Motors' styling chief Harley Earl made one of his sudden direction-changes. Although some new post-war GM cars had fastbacks, most were "bustle-back," with distinct trunks. And rather than having fenders being almost totally absorbed into the car body, GM fenders had distinct shapes, even though they were in low-relief compared to 1930s practice.
Since about half the cars on the road were from General Motors, the 1949-vintage teardrop-influenced designs seemed somewhat out of touch with styling fashion. So sales began to suffer and quick-fixes were put into place until completely restyled cars could reach dealers. Affected brands for 1951 were Nash, Lincoln and Mercury, whose postwar designs debuted for the 1949 model year. (I discussed Mercury styling in the above link, so will not deal with it here.) Hudson's postwar design was launched for the 1948 model year, but didn't get a similar facelift until 1954.
In all cases, the styling fix involved grafting a higher aft portion of the fender with the goal of making the car look less rounded and more squared-off so as to compete better with the broader industry fashion exemplified by Studebaker, Kaiser, Frazer and Ford, as well as General Motors. Packard, which got a rounded facelift for 1948, was totally restyled for 1951.
This is the standard Lincoln that shared its basic body with Mercury. (The top-of-the-line Lincoln Cosmopolitan had a unique body.) Like the Hudsons shown below, it featured what might be termed semi-fastback styling. Still, it largely followed early-1940s ideas as to how cars of the future should be shaped.
The most important changes for 1951 were a reshaped backlight (rear window) and extended, higher fender trailing edges. Seen from the side, the fender modification does "square-up" the appearance a little. But seen from the rear, the design looked more awkward (see the link above for the similar result of Mercury's '51 facelift.)
1950 Nashes had larger backlights than in 1949, but otherwise were nearly identical. Nash came closest to the 1940-vintage teardrop ideal, even to the point of having skirted front wheels. Unfortunately, the car looked heavy, ponderous. It was derided as looking like "an upside-down bathtub."
Not the best side view because of the camera angle. Nash restyled the rear part of the fender, giving it a slight up-kick along with an extension. This reduced the "bathtub" look a little. If the wheel openings had been enlarged as well, the car would have looked much better. Unfortunately, Nash-Kelvinator president George Mason really liked those skirted wheels, and so they remained.
This might be a '52 model because they were essentially identical with the 1953s when seen from the side.
Besides the fender extension, the side trim was revised. Gone was the large chromed strip along the bottom. The ends of the thin side strip lost their ornamentation. Added was a chromed faux air intake shape seemingly inspired by side trim on 1952-53 Ford Motor Company brands. All this create some visual distance from 1948-53 Hudsons, but the design was still stale.