From a BR Training Booklet:

CAV 460/C3 Regulator Control Board
This regulator employs solid state circuitry throughout. The transistors, diodes and resistors are mounted on a printed circuit board contained within a sealed aluminium case which is suitably finned for the maximum heat dissipation. No cut-out relay is required because the rectifier bridge in the alternator prevents current flowing from the battery through the stator when the alternator is stationary or generating less than the battery voltage.

Current control of the alternator is achieved by using the voltage drop across a precisely engineered shunt (integral in the regulator board) carrying output current. An amplifier senses this voltage drop and accordingly produces a switching signal. This signal is used to operate the driver stage of the regulator to control the alternator output by field switching. Voltage control is obtained by measuring the system voltage with a zener diode as a reference and again using alternator field switching to regulate this output. As battery charging progresses, control of output changes from current control to voltage control.