opam lm 350
The opamp appears to be configured as just a comparator. It does not control the regulator. It just seems to control the full charge LED.
However, the location of the "start" switch is odd. Why it is needed and why the opamp's output is shorted to ground, it not clear.
The opamp appears to really be comparing two scaled current. The positive terminal monitors the scaled current through R4. The negative terminal monitors the scaled current through R7. When a discharged battery is attached, the charging current is high, the voltage drop across R7 is high and the negative terminal of the opamp is lower than the positive terminal. When the battery reaches a good charge level, the drop across the current sense falls, the negative terminal exceeds the positive terminal, and the LED turns on.
The charging current is fixed by LM350T output characteristics. The 500 ohm resistor across the LM350T is a trickle charge path for when the battery is fully charged.