The pre-1960 norm was that a restyling for a given model year would be applied to the entire line for the brand in question. But sometimes it didn't work out that neatly. An interesting case is General Motors' Oldsmobile brand for the 1948 model year. That was when GM began to introduce its first post-World War 2 major restyling. Its 1946 and 1947 models were simply facelifted 1942 models which, in turn, were facelifted versions of the 1940 vintage major restyle (production was curtailed during the war by government order).
At that time, GM had three basic bodies (nowadays they'd be called "platforms'). The smallest, "A-body," was used for its lower price brands Chevrolet and Pontiac. The largest, "C-body," was for Cadillacs and higher priced Buicks and Oldsmobiles; lower priced Buicks and Oldsmobiles, along with top-of-the-line Pontiacs used the intermediate "B-body." General Motors restyled the C-body for the 1948 model year and the restyling of the others was set for 1949. This presented no problem for Cadillac, as all its models aside from its low-production limousine used the new body. Buick chose not to use the new body for 1948; it retained its 1947 bodies, opting for the new C-body for its 1949 line. Oldsmobile took yet a different route. Its top-level "98" line used the new large body while its lesser lines persisted with the old B-body.
In theory, this should have been a risky move for Oldsmobile. The new C-body cars were a lot more modern looking than the B-body models, so this might have discouraged potential buyers of B-body cars; why buy something old-fashioned? Actually, the risk was low for two reasons. First, the late 1940s were a seller's market with manufacturers struggling to meet the post-war demand for cars of almost any kind. Secondly, most brands competing with B-body Oldsmobiles weren't offering modern designs either.
Here are examples of Oldsmobiles for the 1948 model year.
During the 1940s General Motors styling vice president Harley Earl saw to it that each of the corporation's brands had strong visual identity, the idea being that a Buick, Pontiac or Oldsmobile could be identified as such when seen from a city block away. In the late 1940s, Oldsmobiles had a cleaner, less-decorated look than their GM stablemates. The cars illustrated above have one or two simple horizontal chrome strips along their sides along with a chrome stone shield on the rear fender. Grilles shared a sort of "fish mouth" shaped opening with fat chromed bars echoing the opening's upper curve. The main difference was that the cars with the older body sported more such bars along with a vertical central bar. Thus the distinctive Oldsmobile appearance persisted even though the new body featured flow-through front fenders, curved windshield glass and other new styling details.