Suppose that you apply to and interview for a position entitled Senior Engineer. All in-person discussions refer to the position as Senior Engineer, but the written offer uses only the job classification, Engineer, without the grade, Senior. The company is glad to meet all of your compensation requirements without negotiation on the company's part, so money is not at issue. After you start, you review your information in corporate HR's systems and find that your effective level/classification is Junior Engineer.
Given that you've already started and have been working for some period of time (weeks, or months, but less than a year) without realizing the discrepancy, how should the situation be handled? Specifically...
Would you be justified in asking that your grade/level be adjusted to the level you thought you were being hired for?
Would you be justified in resigning your position if the company refuses?
The primary interest here is in professional etiquette / courtesy / ethics; "justified" in this sense should be understood to mean "justifiable to other professionals."