This strange device, which you see in this picture, is a synchronizer that has made a real revolution in aviation armament!
Synchronizer allowed to fire without risking to shoot the aircraft propeller. Of course the machine gun was sometimes carried out beyond the screw, but in this case there were problems with its recharging, in addition, the weight balance and its maneuverability worsened. It is for this reason that the synchronizer was created, which was an interrupter, which was built into the trigger mechanism of the weapon. A mechanical or electric breaker detained the firing pin and therefore the machine gun did not shoot the blade. The rate of fire in this case fell, but it was quite a conscious victim.
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On these frames, we see the process of calibrating the device to work with a four-bladed propeller. In this case, an electric lockout system is used. The synchronizer itself is a black box in the middle of the frame, and all adjustments are made here. From it goes the wire to the electromagnetic locking mechanism, located on top of the machine gun. Everything is extremely simple, but it requires adjustment.
I thought it was very interesting, but how about you?
With the best wishes, @gleb-jeglov aka Evgeny!