This is a special design for the MG 34 fixed in it’s bullet-shaped mounting (Kugellafette). They could be found in armoured cupolas or behind armoured plates.

Here you see the optic on top of the mounting.

It has a fixed eyerest which can be adjusted for looking with the left or the right eye.

The “steering wheel” adjusts the reticles arrow according to the distance.

Here a complete set with the case.