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Who manufactured the BH-21 under license in Belgium?
Short answer
Two Belgian companies built it: SABCA manufactured thirty-nine BH-21s under license and SEGA another five, in addition to the aircraft Avia built for Belgium. The order came after the fighter successfully passed the Belgian Air Force's trials in June 1925.

SABCA and SEGA
In June 1925 the BH-21 successfully passed the trials organized by the Belgian Air Force. This led to one aircraft built for Belgium by Avia, while another thirty-nine were produced under license by the Belgian company Société Anonyme Belge de Constructions Aéronautiques, SABCA, and five by the Société d'Etudes Général d'Aviation, SEGA.
The fighter Belgium chose
The BH-21 was a biplane fighter designed and produced by the Czechoslovak manufacturer Avia. It originated as the Avia BH-17 in the early 1920s and was redesigned based on observations from an official evaluation. The BH-17 was a conventional biplane that Pavel Beneš and Miroslav Hajn conceived in those early 1920s in response to a requirement from the Czechoslovak Defence Department for a new fighter. The BH-21 could fly with a payload of 310 kg.
Production and service
In 1925 series production of the BH-21 began, and a total of one hundred thirty-nine aircraft were built for the Czechoslovak Air Force. Throughout its entire service life, the BH-21 did not take part in any recorded combat.
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