My teacher's told us that it is most always one 1920s (boom) and one 1930s (bust) question, and from all the past paper I've seen and all of the mocks that I've done, it's always been like that. Still, I'd recommend to revise all topic areas, or at least 1930s to some extent, because you never know what you might get asked, and you may get completely thrown off by the 1920s questions. Sometimes they can be just mean :/
I wouldn't be /too/ worried, though.
I'm doing America too! What are you hoping to get? For me, I'm praying for a question on change in American society, or something to do with Prohibition!