The historical beliefs of the time period that I believe that this question is asking about are the beliefs about slavery and slave owners. When the novel first begins, Isabel believes that she and Ruth have been freed. Unfortunately, she can't produce the physical documentation that proves her freedom. The consequence is that Isabel and Ruth are sold back into slavery, and they end up being owned by the horrible Locktons.
Madam Lockton is a horribly abusive owner. She dresses up Ruth and treats her like a play thing, and Madam Lockton does not hesitate to physically beat Isabel. While that sounds horrible, Madam Lockton's actions do reflect the beliefs of the time period. Slaves were considered equivalent to property. A slave was something to be bought, sold, and used in the same way that a piece of furniture or an animal was bought, sold, and used. Slaves were dehumanized and not considered to be human; therefore, there was no reason to treat them as such. Madam Lockton treats Isabel like a piece of property to be used and disposed of as necessary, because Madam Lockton believes Isabel is nothing more than a piece of property. This is one reason why Madam Lockton doesn't hesitate to brand and tag Isabel.
Isabel's actions make sense in light of the period beliefs as well. While slavery exists, there are slaves that have been freed. Isabel tries several different ways to obtain her freedom, and she doesn't care which side of the conflict grants her the freedom. When she finally realizes that her best option is to take matters into her own hands, Isabel fakes her freedom papers in order to grant herself something that she was always entitled to from the story's start.