That is an aircraft version of the Spandau (version of the Maxim), standard German machine gun of WW1. This one would have been used by the rear gunner on a bomber or (more likely) an observation plane.
The barrel sleeve is slotted for air cooling since one thing they had was a stiff breeze. No breeze means the motor just stopped
I don't remember if there was a lightened version of the Spandau, besides just not using a water-filled cooling jacket. But the air cooling and the buttstock give it away as an aircraft gun.
Bart Noir