Donar (Thunder) KraftStrahlKanone
Contray to popular reports the KSK (Kraftstrahlkanone) was NOT a crude laser weapon able to penetrate 100mm of armor. It was a crude phaser (PHASed Energy Rectifier) that ran off the Thule Triebwerk - as such, the Haunebu and Vril craft had to hover and divert high voltage energy from the EMG engines to ball cascade oscillators that flowed down to two charged barrels wrapped in precision tungsten spirals capable of absorbing a lot of heat. The power could be changed so it was a phaser weapon. 
On the Haunebu I the upside down looking tank turret is not - close ups show piping running into two elongated 60mm guns that had shrouds over them.
The escape hatch is present because the gunner sat apart in the turret, but this matches NO German tank turret at all, not even the Kugelblitz which wasn't around in 1939 anyway. But because the KSK guns caused both vulnerability and instability, they were dropped in favor of MK installations in the Vril 7 Geist and Vril 8 Odin which had an operating Oberon automated system above the control tower.
Colorized photo of Donar twin 60 mm KraftStrahlKanone (KSK) mounted under the Haunebu I. 
Illustration of German disc phaser weapon with Tungsten transmission coil and ball-cascade oscillators running off Triebwerk
Tungsten transmission coil of German phaser weapon
Part of ball cascade oscillator system for German phaser weapon
Microscopic analysis of tungsten transmission coil
Microscopic analysis of tungsten transmission coil tip
Another version of the “Ray Gun”