I've made some progress by looking at the photo over and over again
There are 4 scales at the central indicator. The most outward scale (the white one) is the relative bearing spanning from 0 to 360 degrees.
Initially I assummed that the 3 innermost scales were somehow used to convert relative bearing to true bearing, but that turns out to be completely wrong, if you will take a closer look at their ranges:
- yellow 400 - 860
- white 800 - 1750
- green 1600 - 3500
Their total range covers values from 400 to 3500, so now I'm pretty sure that these are the sound frequencies! I believe that there must be two pointers here - one pointing the angle and second indicating the cutout frequency.
There are still some questions left:
1. Where is the second pointer? Is that this thin line at the center of the big pointer? There doesn't seem to be any other possibilities.
2. How to choose a specific value with a single pointer, considering that the numbers are spanning through three scales - yellow, white and the green one? Do you somehow switch between three modes? My theory is that the small dial on the left is a three state switch between three ranges and the second small dial is used to choose a specific value, by rotating the pointer.
3. Where to set the volume gain, if there are no dials left? Is that device on the left an amplifier?
http://www.ubootwaffe.pl/images/phoc.../U-995_072.jpg