Polikarpov U-1

Preliminary note: This is some kind of special post as we're covering the Russian copy of the Avro 504K because it has some very interesting backstory.

The Polikarpov U-1 was the Soviet unauthorized copy of the Avro 504K. It all started during the Russian civil war, when am Avro 504K belonging to the British-Slavonic Air Corps (which was sided with the White Russians) crashed near the city of Petrozavosk and the engineer Sergey Ilyushin received the order of going to that zone to recover the airplane and dissasemble it. It was re-assembled again and pushed into service for the remainer of the war, with such good results that another engineer, Nikolai Polikarpov decied to copy it. Under Soviet command, it showed good results, as it served in their different pilot schools from 1921 until 1932 and in some cases until 1935. It also set a new record in the Soviet Air Force as it was the basis for the first experiments with solid propellant rocket boosters takeoff.

It also served in many Asiatic countries. Apparently it served with the Chinese Aviation Ministry, with the Imperial Iranian Air Force and with Mongolia.
The Persians bought them together with some Polikarpov R-1 and R-2, as the United Kingdom refused to sell them airplanes alleging that the terms of the treaty of Versailles forbid the sale of weaponry.
Apparently it also was the backbone of the early Mongolian People's Army Aviation as it served as their main trainer during the 1920s and most probably during the 1930s.

Sources:
1. https://ru.wikipedia.org/wiki/У-1 (translated)
2. http://www.airwar.ru/enc/other1/u1.html (translated)
3. http://defence.pk/threads/imperial-iranian-airforce.138734/
4. http://forum.valka.cz/topic/view/108784