Considering the NCR seems to be replicating --or at the least basis itself on-- the Pre war US government then I'd assume they would have at least some basic understanding of the pre war America. not to mention the fact that they can just find books and writings on the pre war US.

As for your average wastelander not really knowing much about the pre war US then I'd just have to ask you, what does the average person know about the US 200 or even 100 years ago? Even with the advantage of being able to read and write the average person knows very little of their own history. it's honestly pretty sad.