The correct button is fifth from the left.

The inscription states that only one of the six buttons is not mentioned, so exactly five different buttons are mentioned. In the statement of relations between positions, the inscription refers to six buttons; however, the first and last mentioned buttons are the same one.

One now labels the buttons mentioned in the inscription in the order A B C D E A and begins by noting the most precise relation, that is, B is three to the right of C. Since no position may have two letters, our deductions show that from left to right the buttons are D E C A ? B. So the correct button must be the missing one.