I was sure it was a 20mm Hub mount.The BF 109 had 30 mm that was fitted in the propeller hub. A reliable design with a powerful payload the gun suffered from a low rate of fire and problems increasing the muzzle velocity. Pilots compensated by using it at close range where it was very effective. As the war progressed German fighters had additional armament fitted increasing weight, which also drove increasing engine power, with additional weight. When the 109G arrives the aircraft has become almost dangerous to land, requiring full power just to fly at landing speed with the flaps extended.