Oh Rosie, you must ask Ma!

The odd positioning of the stamp on the back of this card may have been part
of a complex language of stamps. Senders would send a message such as "I
love you" by positioning a stamp at various orientations and locations on an
envelope or card.

This "language" has been largely forgotten today with our automated postal
machinery. One of its few remnants may be in the custom of putting White
House stamps upside-down to show disdain for whichever president is in office,
a practice that goes back to at least the Reagan administration.