On the one hand, the fashion of visual streamlining yielded what were called "fastbacks" where the roofline fell off to the rear in a smooth curve that ended near the back bumper. On the other hand there were "bustlebacks," where the trunk area formed a curve separate and distinct from that of the roofline. As cars grew longer, bustleback trunks became increasingly large, offering considerably more storage space than fastbacks could manage. Even in the early years of the fastback fashion, bustlebacks were more practical from a storage standpoint, so all that fastbacks had going for themselves was a sleeker appearance. By the early 1950s, GM dropped fastback bodies from its product lines due to poor sales levels.
Because of all this, along with an awareness of the usual appearance of pre-1930s sedans, I've tended to think of bustlebacks as appearing on the styling scene as of around 1940. But I was mistaken. I failed to take into account what is revealed in the Gallery below.
Here are examples of post-war GM cars with the two styles.