Analysis of information sources in references of the Wikipedia article "Bofors 40 mm L/60 gun" in English language version.
The big breakthrough came at firing tests in Belgium in 1935 against a British competitor. It was found that the Bofors gun could be moved more than twice as quickly as the competitor's gun and that it scored three times as many hits when firing on aerial targets. The Belgian officers were amazed. The demonstrations in Belgium took place in the presence of representatives from the French War Ministry. It soon resulted in an order from the French Army, which was quite remarkable. Traditionally, France bought no guns from abroad.
In March 1932, the official trials of the automatic antiaircraft cannon L60 were completed, and it was put on the market. In 1933, British and American military engineers studied the gun, and were quite impressed - the Swedish system was more effective than the obsolete British Vickers "pom-poms" and had more firepower than the "Chicago pianos" used by the US.