Okey dokey, that's all very helpful.
To recap, then, 4 and 7 are probably some kind of common ground, and in either case are a reference voltage we can call ground or zero for measuring the voltages at the pins adjacent to them.
At any given time t, using pin 4/7 as a zero reference voltage, V3 = - V5 and V6 = -V8
Furthermore, measuring the voltage (for a time period, obviously, rather than at an instant) across pins 3&5 (not the ground) will reveal an AC voltage of whatever that badly printed value is, and will remain that voltage with a current draw of 0.3A.
Measuring the voltage across pins 6&8 (not the ground) will reveal an AC voltage of 20.5 volts, and will remain that voltage with a current draw of 0.65A