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In page Henschel Hs 129:

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In May 1944, a Hs 129B-2, Werknummer 141258, was fitted with a mock-up of the BK 7.5 and underwent aerodynamic testing at Travemünde. The gun fired a 12 kg shell and in an emergency the entire installation could be jettisoned.[3] During August of that year, the first three HS 129 B-3s commenced trials.[1] While deliveries commenced to actual frontline units, around 25 B-3s are believed to have been completed prior to the production line being permanently shut down in September (a small number were reportedly also created by converting B-2 aircraft). In the field, the B-3 proved to be effective even against members of the IS tank family,[1] however, its small numbers meant that it had little effect on the war effort.[citation needed]