The title image as a symbol for government oppression
The image of a iron boot is pretty clear symbolically. Metaphorically, being under someone's foot is a common way of saying that you're under their control. The 'iron' aspect of the image is also standard imagery. Think "Iron Curtain."
The Anthony Meredith narrative as an allegory for time
By focusing 700 years in the future, and asking the reader to interpret the Avis Everhard narrative through the lens of a historian with no personal attachment to the events, London creates an interesting tension around the idea of time. He latches the story to its position in history, and he makes the reader remember how fleeting the present moment is. So Anthony himself can be seen as a hero of time.
The Avis Everhard story as an allegory for paranoia
Paranoia is one of the most excruciating types of suffering, and it's difficult to capture it in art, but London does it well through the character of Avis Everhard. The idea that the government could be monitoring your location and your actions through espionage helps to create the same tension that paranoia would cause, and Avis becomes paranoid. However, London reminds the reader through the last line ending mid-sentence that for Avis, the paranoia was actually correct, since she really was in danger.
The motif of oligarchical tyranny
Another important aspect of paranoia symbology is the aspect of conspiracy. Historically, the dystopian genre has used tyranny as way to criticize totalitarian monarchies or dictatorships, since American philosophy was founded against those forms of government, and the American fear of one person with too much authority is still a real part of American politics, but in the novel, London asks the reader to consider the idea that perhaps the real danger to capitalism isn't communism, but oligarchy.
Oligarchy does two things. Firstly, it makes the reader criticize their assumptions about the free market, and secondly, it establishes another important kind of paranoia—the fear of conspiracy.
The image of a historical document
Most of the novel's plot is offered by way of a symbol, that of the discovered archaeological relic. This is a strange lens to view the early 20th century. Even today, those years are 100 years ago, but they don't feel 'ancient,' yet. London makes the reader remember that one day, the current era will be ancient history.
