This optic was used on the machinegun MG 08, which was first introduced in World War I and was still in use in many German defence lines in the 1930s.

This one was made by Busch. The optical value is 2,5×20.

This knob corrected the distance by leveling the reticles arrow. The scale is in hm, so times 100 meter.

This one, with an extension tube, was found on Ebay.