What did the narrator know about the bank?

As subsequent events demonstrate, the narrator doesn't know much about how banks work. All he knows is that he gets incredibly nervous whenever he enters one. The clerks, the little windows, the sight of the money—all of these things make him nervous. The narrator's evident nervousness makes him an object of curiosity to the bank manager. There's something about this strange customer's antsy demeanor that makes the manager think that he has a Pinkerton detective sitting in front of him rather than just a nervous old man who hates banks. But once he's been assured that the old man is not a detective after all, he assumes that he must be an eccentric millionaire instead.
However, once he finds out the truth, the manager's whole attitude suddenly changes. Now he acts coldly towards the narrator, feeling that his time's been wasted. The attitude of the bank clerks and the accountant isn't much better. When the narrator set foot inside the bank, he didn't know much about them and how they worked. But he does now. He's had a very educational experience indeed.