AUTO VOLTAGE REGULATOR
"Field" excitation current is supplied by T2 (BD902) biassed on by R3; to rotor winding.
As circuit voltage increases zener diode (& series rectifier diode) D1 AND D2 will conduct, causing T1 to switch on.
This will rob base current from T2 causing it to switch off and remove exciting current from rotor.
This is the voltage high limit which represents a full charged battery state.