Incoming!

[Mush Martin]

I imagine it listened for the common frequencies of highspeed
screws and wouldve likely had three hydrophones

I further would suppose that the display would not be
dissimilar to the depth charge direction display on later
US subs. ie. a circle of lights eight/sixteen? that represent
the circumference around the ship and the hydrophones
registering the highspeed sound would light the bulb on
the indicator in the appropriate quarter of the display

thats all a guess.
M