A "Cinderella Stamp" is a stamp that was not issued by any postal authority, but was not meant as a counterfeit. Most are issued by whimsical stamp enthusiasts in the name of non-existent countries. In the episode title's meaning, however, the Cinderella stamp is literally that - a stamp that makes one a princess!
Also known as "The Little Princess of Darkness."
This episode marks the beginning of the series' second opening and ending themes and animation sequences, vocal versions of the first opening and ending themes, which are the only vocal versions done for this series.