H. Rayner Wilson's 'Power Railway Signalling' has a description of the original signalling on this line (Pages 180 to 183) (Part 1B if you have the Peter Kay reprint)
The equipment was provided by Spagnoletti and Co. Two aspect colour light signals were provided (with the red aspect above the green). Signal boxes were provided at all the stations. However to quote: 'Signal boxes with attendant signalmen, are provided at each station. These men are not necessary for signalling purposes, and they do not interfere at all with the operation of the signals, but they are useful for booking the passage of the trains and watching that all is right, whilst their presence would be extremely useful in case of emergency. All the relays and operating mechanism, too are in the signal boxes so that the men keep them under constant observation.'
Train detection was by means of track circuits, reinforced with what are described as treadles located between 350 and 400 feet in advance of signals. These worked by completing and electrical circuit when they came into contact with a special brush mounted on the rear vehicle of the train.
Where points existed (Finsbury Park, Drayton Park and Moorgate) these were worked mechanically from lever frames, these frames also controlling the signals in the area.