PV-1 aircraft machine gun adopted to AAMG use after it was retired from aircraft use. Wikipedia even has a picture of the triple mount from a different angle:
http://en.wikipedia.org/wiki/PV-1_machine_gun
Notice that the middle gun has a similar barrel jacket to the picture you posted and different from the other two; probably this gun was made with at least the two different jacket styles.
Obviously this triple mount made a lot more sense than the extremely heavy quadruple AA mounting of the regular water-cooled Maxim.
Not an easy one, since it's easy to forget about the relatively mundane pre-WW2 PV-1, and I also did not know that they were adopted to ground use during the war.
Re: ??
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