I have my answer From wikipédia : "armament, an innovative rear-gun emplacement designed by the Ikaria-Werke: a rotating conical rear "turret" of sorts, manually rotated with a metal-framed, glazed conical fairing streamlining its shape, with the open section providing the firing aperture for either a single or twin-mount machine gun at the unit's circular-section forward mount"
I am happy ! Thanks