This cover from Poland depict a se-tenant joint issue with Croatia on motive "Sources of Salt". The "4 zt" stamp on the right corner shows a picture of the
Bochnia Royal Salt Mine in Poland, one of the oldest salt mines in the world and also the oldest commercial company in Poland. The Bochnia salt mine was established in 1248 after salt had been discovered there in the 12th and 13th centuries, and became part of the royal mining company. In 1990 the mine ceased producing salt but remains a tourist attraction. In 1981 the Bochnia salt mine was declared a heritage monument. It is one of Poland's official national Historic Monuments, as designated on 6 October 2000, and is overseen by the National Heritage Board of Poland. In 2013 the mine was added to the UNESCO World Heritage List as an extension of the Wieliczka Salt Mine inscription of 1978.
