Avia Bk-534, German users

The German Third Reich used the Avia Bk-534 mainly for training purposes but also for some testing about the hub mounted cannon.

Apparently some Bk-534 were also used to test arrester hooks for the aircraft carrier Graf-Zeppelin in 1940 and 1941. The tests showed that the airframe wasn't ready for rough carrier landings.

Sources:
1. https://en.wikipedia.org/wiki/Avia_B-534
2. http://surfcity.kund.dalnet.se/avia_germany.htm
3. Salamander Books - The Complete Book of Fighters