Hmmm....
A 360 has a permanent magnet alternator rather than the electromagnet alternator of the SOHC fours and is quite different.
On a 360, 3 wires come from the alternator stator - pink, white & yellow. Pink connects to one side of the rectifier. Yellow connects to the other side of the rectifier. Together, they supply the full-wave AC to the rectifier which then converts this to DC. Yellow ALSO connects to white ONLY when the headlight is turned on. Thus, the white is added to the yellow and sent to the rectifier. Yellow ALSO connects to the regulator, which we'll get to in a minute.
The rectifier has one connection to ground (the green wire) and one connection (red w-white stripe) which goes to the solenoid where it meets with the big cable from the battery (+) and also with the red wire that supplies the key switch and all the rest of the bike.
So, pink & yellow (plus white) supply the AC to the rectifier. DC out from the rectifier on the red w-white to the starter solenoid / battery / bike load.
Back at the regulator, it has the yellow we discussed, a green ground wire, and a black wire which is the battery voltage downstream from the ignition key. The regulator "sees" the battery voltage on that black wire. If the voltage there gets too high, the regulator shunts the output from the yellow wire right to ground, meaning that the yellow wire isn't supplying the rectifier any more. Thus, the battery voltage drops and the rectifier "sees" the drop and stops shunting to ground. And so on...
So, if you run the motor with the alternator disconnected from the rectifier, you should see some large AC voltage between the pink and the yellow. I dunno what, but 21 VAC seems maybe a bit low...
Next, unplug the regulator so it cannot shunt anything to ground. That takes it out of the equation, and for testing, you won't need to worry about overcooking the battery. Don't run the system without the battery hooked up, even if the rectifier output (red w-white) is hooked to the solenoid and the rest of the load.
You can test the battery voltage as you have done, with it all hooked back up (except the regulator) and you should see 13 ~ 15 volts across the battery.
You can test the rectifier by using a diode tester. There are 4 diodes - they each must conduct one direction but not the other. Unplug the four wires. Test between Y-G, Y-R, P-G and P-R.
Finally, or rather, this should've been FIRST - the battery must be good and capable of ACCEPTING the charge. If it has a dead cell or is permanently drained, you'll NEVER see a proper voltage at the battery.
Good luck,
Kirk