After Dean leaves Marylou and Sal in California, they spend a night in a hotel. In the morning, you can see a ring on Sal's left hand (probably a ring that belongs to the actor, but has nothing to do with the character or the movie).
In the opening scenes, Sal Paradise hitches a ride on the old farm truck. The large, round hay and straw bales in the background weren't available until 1972, when Vermeer built and sold the model 605 baler. Even then, the bales were much smaller and looser until the late '70s or early '80s on United States farms.
When Sal leaves for Denver in 1947, he crosses the George Washington Bridge, which has upper and lower decks. The lower deck was added in 1962.
Among Sal's books is a copy of Saul Bellow's "Seize the Day", which was first published in 1956.
Set in 1949, but the characters ride in a 1950 Hudson.