And here's an article:
AVRO
The AVRO 504K two-seat biplane first appeared in Russia during the 1920s, when some machines entered service in the Don air detachment of General Wrangel's White forces and the 2d air detachment of the White Volunteer Army. They took part in anti-Bolshevik fighting in South Russia. Some were captured by the Reds. In 1922, Soviet Russia bought a batch of AVRO 504Ks (on wheels) and AVRO 504Ls (on floats) from Great Britain. Soviet factories copied this plane as the U-1 (AVRO 504K) and MU-1 (AVRO 504L). The U-ls served as military trainers until 1932. Civil air clubs flew U-ls until 1935. Thousands of Soviet pilots were trained on this type. The first U-ls were built at GAZ-5 aircraft plant. Beginning in 1923, production was transferred to the "Red flyer" plant at Petrograd. Production ceased in 1931 after 664 U-ls and MU-ls had been built. In 1931, takeoff rockets were tested on a U-1 by S. Mukhin. The MU-1 seaplane was built from 1924 to 1930, with seventy-three delivered. As a military seaplane trainers, they served until 1934.