From an 1890 edition of the Szarvas és vidéke, a weekly Hungarian newspaper, an explanation of the "stamp code" used to signal one's intention when sending mash notes and such through the Emperor's post.

The secrets of the language of stamps. For all those who are in the situation of Hero and Leander, and similarly to them can only exchange secret signs about the feelings of their hearts, here we publish the secrets of the language of stamps. If the stamp stands upright in the upper right corner of the card or envelope, it means: I wish your friendship. Top right, across: Do you love me? Top right, upside down: Don’t write me any more. Top right, thwart: Write me immediately. Top right, upright [once more again???]: Your love makes me happy. Top left, across: My heart belongs to someone else. Top left, upright: I love you. Bottom left, across: Leave me alone in my grief. In line with the name: Accept my love. Same place, across: I wish to see you. Same place, upside down: I love someone else. – We hope that besides the inventor of the “new language” there would be other persons too who would eventually use it.

Poemas del río Wang: The language of stamps (via Neatorama)