Claire's Pregnancy
In her life with Frank during the 20th century, Claire wanted a child but was unable to conceive. Frank then refused to adopt a child with Claire. However, when she travels to the 18th century, Claire's dream of getting pregnant is realized once she falls in love with James. Claire's pregnancy is symbolic of the beginning of her new life in 1743 and the wildly different circumstances she found herself in.
Red Coats
Red coats are one of the most recognizable symbols in the world. They are symbolic of the British military, which features heavily in the novel. James spent much of his life fighting against the British; the red coats are also symbolic of the oppression of the redcoats and the British government.
The Vase
The vase that Claire buys is symbolic of the stability that she thinks she will feel in her marriage to Frank and in her life more generally.
World War II
World War II, which Claire had just been a part of and lived through, is symbolic of humanity's tendency to destroy themselves and fight amongst themselves. Claire sees this trait in humans all the way back to the 18th century, where she sees humans fighting amongst themselves yet again.
The life of a woman
Claire's experience in Scotland in the 18th century is an allegory for what her life as a woman would have entailed. In the 1950s, she would have had access to modern amenities, but her life would be strikingly similar to the life she would live in the 18th century otherwise: she would have to stay at home and bear children. Things, in other words, don't change too much from century to century.
