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Please clarify: is the engine running for all these measurements? How does the engine run if the power switch is off? Your wiring must be really odd; normally, the power switch disconnects the coils from the battery and R/R when off, so I don't know how this is possible.

The regulator/rectifier requires a battery be present to control the voltage, so disconnecting the battery will cause the voltage to rise, just because there is no load to soak up the power. If the battery voltage drops to 10.5V under load, it probably has a bad cell; have it tested. The regulator may not be able to keep up with the load at idle if the battery is bad, so may not be bad. It works by shorting out the alternator stator when the battery voltage rises above the set point (around 14V, but it varies).
 

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OK, so the switch just turns the lights off; is there another switch to turn the ignition off?

With the engine running, battery connected, the battery voltage of about 13V is normal; if you rev the engine, it should rise some. The engine, alone, draws about 2-3 Amps, on average, operating just the coils, assuming you have 5 Ohm coils.

When you switch the lights on, and the voltage drops to 10.5V, I'm guessing it is at idle. If you increase the rpm, does the voltage rise with the lights on? With the lights on, you are adding a much bigger load, and the voltage drops, indicating the battery can't support the load, and the charging system, alone, can't either. The charging system of that vintage a bike may let the battery drain at idle, and not begin charging until about 1500 to 2000 rpm; it cannot overcome a weak battery. A voltage of 10.5V indicates 5 of the 6 cells in the battery are working, and one is not, or the battery is really drained. Again, have the battery tested, or replaced, as the measuresd values indicate it is weak. Even with the switch on and the engine not running, the voltage should be above 11.5V with a good, fully charged battery. Once you have a known good battery, try again; if the voltage drops below 12.5V still, the charging system is the reason the battery you have is weak. Check the connections between the stator and the rectifier/regulator, as these often overheat and stop conducting enough current.
 
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