zenith: (Ch= 1001); 935
I would think the tuning indicator is a 'Tuning Eye tube'...But I don't see one listed on specs of radio. From your picture, there should be a tube turned side-ways, inside a small housing with resistor . I hope this helps...let me know.....AL
The mechanism(?) is housed in a small trapezoidal box with a light behind to project on to the small screen in front. The indication is in the form of a shadow that varies in widh, becoming narrow when a strong signal is received. The shadow moves when the device is tapped.
The circuit diagram shows the device in series with a resistor in the anode circuit of a 76 triode, but there is no hint of how the thing operates. I just enlarged the legend - it says "shadowgraph".
The display certainly is reminiscent of the much later EM84.