Original Rundblick Fernrohr 16 German artillery gun sight. This is designed for indirect fire. Typically the optic would be sighted off of either an aiming stake, aiming circle, or the red potion of another artillery sight. It is a little bit dirty but in good condition with much of the original paint. Both levels are still working. The optics and pretty clear and the reticle illumination works well.
Essentially the aiming process works like this:
Step one, calculate the range and direction.
Step two, input the appropriate elevation and direction into this sight.
Step three, traverse and elevate/depress the gun until the optic lines up with the aiming stake or other aiming point, like the red portion of the Rbl.F.16 of the lead gun in the battery. (That's what that red painted portion is for.)
Step four, fire and adjust.