The I-70 series tuning indicators were used in radio compass control boxes, particularly the BC-434 and C-4/ARN-7. Their prupose is to show the navigator or pilot when the loop antenna of the compass set is pointed directly at a tuned-in station. The direction is shown on a separate indicator, which allows the aircraft to find it's location relative to two or three stations. At night or above clouds, this is an essential piece of equipment before radar was common-place, and could guide bombers directly to German targets using German radio stations.
There are early and late versions which have different rear casings, but identical electronic properties and faces. Note that the top right of the indicator has a location for a small screw-in special type panel lamp. These are rare to come by today.