Karabin maszynowy obserwatora wz.37

The karabin maszynowy obserwatora wz.37 (Polish for "observers machine gun") is a Polish version of the Browning wz.1928. It was a flexible machine gun used in the some Polish airplanes in the beginning of World War II.
History.
In mid-1930s Polish small arms designer Wawrzyniec Lewandowski was charged with developing a flexible gun based on the Browning wz.1928. The desired changes included raising the cyclical rate of fire to 1100 rds/min, replacing the buttstock with a spade grip at the rear of receiver, moving the main spring under barrel and, most importantly, changing the feed system. The gun's original 20 round box magazine was impractical with the gun's high rate of fire.
The solution was found in developing a new feeding mechanism, which was added as a pack to the standard receiver. It contained a spring-loaded lever, which when cammed by the lock during locking would grab a round from a 91-round pan magazine located above the receiver and force the round into alignment to feed during unlocking.
The weapon is the world's only specialised aerial flexible machine gun based closely on the Browning M1918, or "Browning Automatic Rifle".
The wz.37 was dubbed "Szczeniak" (Polish for pup) due to its lightness and compactness compared with previous Vickers E and Vickers F machine guns. The wz.37 was used mostly in Polish PZL.37 Łoś bombers.