Which letters are used to identify components is a decision made by the designer or draftsman, there is a convention but not a definitive standard. In your case 'G' is a Varistor to sink over voltage spikes.
I have designed a similar circuit myself where it was necessary to 'hold up' a microcontroller supply when the incoming AC failed. It had to reserve enough power to operate solenoids after a power outage. I did it by using a large bank of capacitors then a single diode followed by another capacitor. The idea was the second capacitor couldn't discharge backwards to the solenoid supply because the diode bloacked the flow. It would be unusual, but maybe the second bridge is being used in the same way, making sure the "To circuit board" point on your diagram can't backflow if something pulls the voltage across the 20 capacitors down low. Possibly it is cheaper for them to use another bridge than stock an additional component type, even if most of it isn't used.
Brian.