Lorenzo,
The gun seems to be an original Swedish 1936 Model. You may have noticed that there is no apparent sighting system, but this could mean that the Gun is controlled by a Predictor. This would work out where the Gun should be pointed to the 'future position' of the aircraft to hit it. This position would vary with Speed, Direction, Altitude, and Angle of Flight of the target.
The Predictor would move pointers on dials in front of the layers (not visible in the photo) and the layers would elevate and traverse the gun to match the pointers, without looking at the target. When the pointers were matched, the Gun would be fired.
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George Cross Island
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