i'm an old fart, but i worked on a lot of those in the 60s-70s. DISCLIMER: i'm digging this out of my distant memory in a foggy part of my brain.
if you take the cover off of one of those regulators there will be 2-3 sets of points under there, the points will make and break contact by way of coils of fine armature wire under the points (they are like mini-solenoids). there are 4 terminals (or a plug with 4 wires) thes sense battery voltage- B+, ign on/ off-i terminal, a field terminal-F, and an idiot light terminal.
all of the old (actually ALL) of the alternator systems work the same way, when you put battery voltage to the field the alternator will put out full output-this can be 16+ volts. when you disconnect the battery from the field the alternator will put out 0 volts. voltage can be varied by how fast the points cycle in the regulator. the actual figure you are looking for with a CHARGED BATTERY is 13.8-14.2 volts. with a low battery the voltage will be considerably higher, right after start the voltage will be higher, the voltage is regulated by the the voltage input by the 3 out of 4 terminals on the voltage regulator and the time differential of open/closed points between the battery and field.
there are 2 common failure modes on a mechanical regulator"
1-a break in the fine armature wire that controlls the points, this will result in 0 charge.
2-pitted points in the regulator, this usually results in the points sticking and you will get a 16+ volt charge, or just erratic readings.
checking for a bad regulator, you do that indirectly:
1-fully charge the battery and hook a voltmeter to it, it should read 12.5-12.8 volts.
2- start the car and look at the voltage on the voltmeter, it should read 13.8 14.2 after stabalizing.
3-if it reads around 16, replace the voltage regulator.
4-if it reads 12, unplug the regulator and jump from the battery to the field terminal.
A-if the voltage jumps to around 16 replace the voltage regulator.
B-if the voltage stay at 12 replace the voltage regulator and alternator (you could just replace the voltage regulator but if the regulator was allowing the alternator to charge at 16 V and it burned out it will damage the new regulator.
these are just the rules for mechanical regulators and have no correlations to todays charging systems.
the plug i referred to above was for ford regulators, they were marked B-2-3-4, you would use a (floyd) cotterpin to jump between the B and 3 terminals to supply bat voltage to the field terminal.
even going further back, to the days of generators, does anybody remember those little instruction sheets that said you had to polorize the generator by useing a jumper and strike a spark between the B and A terminal???