In the years leading up to the U.S. entry into World War 2, General Motors marketed limousines by Buick (the Limited, model 90) and Cadillac. Thereafter into the 1950s, only Cadillac had production limousines in its Series 75 line.
From the 1940 through the 1956 model years, Cadillac offered only two different limousine bodies, whereas there were four generations of its basic car lines. As a result, Cadillac limousines were often stylistically out of phase with the regular line even though many ornamentation cues were incorporated where possible. For model years 1957 and 1958 it appears that the greenhouse of the 1950-vintage limousine was grafted onto regular Cadillac bodies.
This advertisement shows the limousine body introduced for the 1940 model year.
It was given the same '41 facelift as the rest of the Cadillac line.
Regular Cadillacs were completely restyled for 1948.
But the 1940-vintage limousine body was used through the 1949 model year. No attempt was made to facelift it to mimic 1948 styling cues.
The Cadillac line was restyled for 1950.
So was the limousine, as can be seen here in its 1952 guise.
The Cadillac line was restyled for the 1954 model year. The limousine body in the passenger compartment zone appears to have been carried over from previous years.
Cadillac was restyled for 1957 and facelifted in 1958.
The 1950-based limousine upper body continued through 1958. Compare the passenger compartment roof and windows to the 1952 limousine shown two images above.