what does the term "hawker hurricane in yugoslav service" mean?
The ser vice of Hawker Hurricane Mk I fighters in Royal Yugoslav Air Force.
Hawker Hurricane in Yugoslav service
In late 1937, the Royal Yugoslav Air Force (, VVKJ) placed an order with Hawker Aircraft for twelve Hawker Hurricane Mk I fighters, the first foreign purchase of the aircraft. 
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Hawker Hurricane in Yugoslav service [SEP] Introduction
In late 1937, the Royal Yugoslav Air Force (, VVKJ) placed an order with Hawker Aircraft for twelve Hawker Hurricane Mk I fighters, the first foreign purchase of the aircraft. The Royal Yugoslav Air Force (VVKJ) operated the British Hawker Hurricane Mk I from 1938 to 1941. Between 1938 and 1940, the VVKJ obtained 24 Hurricane Mk Is from early production batches, marking the first foreign sale of the aircraft. Twenty additional aircraft were built by Zmaj under licence in Yugoslavia. When the country was drawn into World War II by the German-led Axis invasion of April 1941, a total of 41 Hurricane Mk I's were in service as fighters.