From Bulgarian: Kingdom of Bulgaria.
After the Russo-Turkish War of 1877-1878, the 1878 Treaty of Berlin set up an autonomous state, the Principality of Bulgaria, within the Ottoman Empire. Although remaining under Ottoman sovereignty, it functioned independently. The new country introduced its own coinage in 1881, displaying the legend "Principality of Bulgaria"; as a nominal vassal of the Ottoman sultan, the monarch of Bulgaria was styled as a Prince.
Full independence from Ottoman control was declared in 1908, whereupon the monarch became a Tsar (king) and the country became ЦАРСТВО БЪЛГАРИЯ (Kingdom of Bulgaria). The word Tsar is abbreviated fro "Tsesar", a Slavic rendition of Ceasar; it formerly meant "Emperor" during the time of the Second Bulgarian Kingdom (1185 - 1396), and was used in this meaning by the emperors of Russia. In modern Bulgarian context though it is equal to "King".
The legend appeared on coinage for the first time in 1912.