Pieniński Park Narodowy is a protected area located in the heart of the Pieniny mountains in the southernmost part of Poland, the oldest national park in Poland.  This item has the right size and appearance to be a postcard, but the reverse is filled with (Polish) text and has a serial number, so it appears to be an entrance ticket.  However a determined person could mail this like a postcard if they were so inclined.  Grade: 1