Brunetti Maranello output transformer wiring

[Zouto]

Hi, I'm looking for information about output transformer wiring of a Brunetti Maranello.The secondaries are green (16 Ohms, speaker output), black (gnd) , red (not connected), red(to presence cct.? on pcb), black (gnd on pcb).So what's the use of two separate secondary ground taps?
Schematics would be helpful but seem to be not available.
Thanks for hints and informations
Zouto