MICKY

I've never seen this noted, before.
Depending on whether you had a HEATER installed or not...the vehicle took ANOTHER higher capacity generator.

It wasn't the regulator that was "low cut in" but the generator was designed to produce power at a lower speed.. Cut in is determined by when the generator armature voltage meets or exceeds battery voltage, which is a function of winding design. It could be as simple as a pulley change to spin the armature faster, but more likely (due to a generators max speed) was done with the coils.
My '41 K-7 came with a 30 amp unit, there was a 42-46 amp optional and a 50 amp special equipment available, My '48 came equipped with the rare 50 amp generator, which was much bigger physically and was also made in 50 and 55 amp versions in 12 volt and was the mainstay of generators used in heavy trucks until alternators became std equipment.

OK. Generators have three unit regulators, right? Voltage cut out, current cut out and the shut off part?
I assumed "cut in" was a special regulator.
The 50 amp capacity is funny, now. There are everyday vehicles running around with 200 amp alternators. Mostly we see 100 amps or so. We have a Snap On VAT45 and I check alternator output regularly. 100 amps is achieved easily. And these vehicles seem to use it all at some point!
I have some of those large generators you mention. Yes, they came off larger trucks.

The "cut out" is essentially a relay that is tuned to voltage and current. It has two windings on the bobbin. The 1st is many turns of fine wire and responds to voltage and is wired across the armature to sense voltage, once the armature voltage, is slightly above system voltage the relay closes, and the 2nd winding come into play, it is a current winding and all the current from the armature passes thru the winding. The voltage and current windings on the bobbin are wound so when current flows from the armature the magnetic flux from the two winding complement each other and keep the relay closed even if the output voltage drops. As long as current is flowing from the generator to the system, the relay will stay closed, but if the current reverses, and the battery trys to drive the generator, the flux from the two winding fight each, canceling each out and the relay opens.
So cut in is determined by armature voltage, which is a function of design, speed and field strength. You could wind an armature that had more turns of finer wire, and the voltage would be higher at a lower speed, but you would sacrifice maximum current output. Another way would be to add more poles, most automotive generators are 2 pole but 4 and 6 were made. A 3rd way would be to wind the field so it was stronger, but that risks overheating the field, lastly you could change the drive ratio, increasing the generator speed compared to engine speed, But there is a maximum safe speed for the commutator, and spin too fast and the brushes will tend to bounce on the commutator, and in a worse case the commutator may start to come apart from centripetal forces.

OK. Generators have three unit regulators, right? Voltage cut out, current cut out and the shut off part?
I assumed "cut in" was a special regulator.
The 50 amp capacity is funny, now. There are everyday vehicles running around with 200 amp alternators. Mostly we see 100 amps or so. We have a Snap On VAT45 and I check alternator output regularly. 100 amps is achieved easily. And these vehicles seem to use it all at some point!
I have some of those large generators you mention. Yes, they came off larger trucks.

Air cooled automotive alternators generally can't run 100% duty cycle, draw any more than 30% of rated output (continuous) and they will fry. That is why we see alternators in the 100-200 amp range, so they can provide 30-60 amps of continuous output.
Battery will only accept current at the rate they can handle. putting a 200 amp alternator on will not charge the battery any faster.
I charge 620 amp/hr battery pack with a 75 amp alternator. After running a Espar heater all night, when the truck is 1st started I will see a peak current of 75 amps, within 30 sec it has dropped to under 50 amps, and within a minute is less than 30 amps, where it will do the bulk of the charging, taking a full 10 hrs to recharge the battery pack. 100, 200, or 300 amp alternator wouldn't make any difference after the first 30 sec or so, the batteries internal resistance determines how fast and how much current it will take.
I use a Delco 25SI 75 amp unit. It WAS designed to be able to supply close to 100% duty cycle and is 4-5 time the size of auto 100-200 amp units. It is also brushless design and has a large heat sink and serpentine air flow to cool the unit.
The Delco DN 50 is a 200 or 300 amp continuous unit used for motor coaches and is OIL COOLED! where the windings and diodes are in a oil bath.

CB, we see alternators putting out CLOSE to their stated capacity (system tests GOOD at Autozone and O'Reilly's) and yet the battery is going low. Our VAT-45 (and our VAT-40 before it) finds the problem. AZ and OR don't have $3k charging system checkers, usually.
Something is wrong with your scenario?
It seems that many modern vehicles are on the bleeding edge of using every bit of power from the alternator.
With all these circuits, engine off power draws are relatively common--we fix those.. we can sufficiently load a battery to check it..so it's OK, however, the "100AMP" alt can't keep up. It's puzzling.

Can't speak to what you are seeing as I'm not there to see it.
I 100% guarantee you try an pull rated output off a auto alternator for an hr or more it will die! It just doesn't have the thermal mass to cool the diodes, the brushes don't have the area to transfer the field current at full power continuously.
It is very easy to monitor what the running current is with a good VAT, just put the pick-up on the alternator output wire.
Simple things like one diode out on a trio that feed the field (these are different than the main output diodes) will see a significant reduction in the charge voltage and current under load heavy, but little or no reduction under light load.
I HATE the modern Vat's where vehicle and alt info has to be entered for them to work. I have an older workhorse Snap-on vat, that uses a inductive pick-up for current and diode checks, but a manual carbon-pile for load (I don't remember the model number).
Even with all the motors and electronic, running loads of over 60 amps, continuous would be impossible to reach, I could see 40-50 amps with A/C, fuel pump and injection ABS etc but not over 60.
The peak current, could reach the alternator maximum, but not the continuous load. It is why a 30 amp 6 volt generator could be enough for an old truck, it produces the same power as a 15 amp 12 volt charging system, you lawn mower likely has a bigger chargeing system! Generators are limited to the maximum they can handle at 100% duty cycle, a 30 amp generator can handle 30 amps all day (or night) long. With the increase in continuous load, an increase in max current had to be to keep the alternator living. They could just increase the duty cycle, but that requires increasing the size and area of the whole unit, and 75 amp continuous doesn't sound as good to Joe consumer as 230 amp! Look at the size of these car alternators, they are smaller than the 37 amp units that cars had 30 years ago.
With PMGR starter, and high under hood temps, batterys failure is common, and can be fairly weak and still crank the engine to start. Which is why it is more common to have a sudden "no crank" than a car that cranks too slow to start now a days, the battery has been failing for months, but without the warning signs that the old wound field starter motors used to give. With so much electronic, there is a whole lot that is connected to un switched battery supply, and all it takes in one thing putting a small draw to pull a battery down over night, add to that a short run time, and the battery never gets re charged, just a surface charge. The cycle of slow discharge and not fully re charging goes un noticed until the car will not start.
A lead acid battery pulled to 50% will take 10 hr or more to fully recharge, most cars are not driven that long, but if everything is working correctly, the battery never gets pulled down to lower than 90%. so if there is a parasitic drain, and it is not caught early the battery will never be fully charged, and will be permanently damaged.
On my truck, I have the equalivant of a VAT on board at all times, I have an ammeter that I can see a draw as low as 1-1.5 amps and as high as 100 amps, and a volt meter, the two together tell me all I need to about charging and battery health.

https://www.youtube.com/watch?v=GPJao1xLe7w
This is not recommended but shows how low the parasitic draw should be, You shouldn't have enough draw to kill a capacitor over a weekend. Even moisture and dirt on top of a battery can put a draw on it. You may not see it much in dry climates, but here you can put a volt meter on one terminal, and the other on the plastic case and get a reading!
I use a Maxwell system (one made by Maxwell for starting trucks) and one "battery" of Maxwell replaces 3 lead acid batterys. I still have 3 deep cycles for my Key off loads, which are high.

Fooling around with a very reluctant to move C-30, today.
Future parts for C-1 and Mack ED.
Everything gets saved, of course.
I was hoping to get it closer to the shop but "wheels came off" here. Oh, well.
This truck is from that "famous" estate sale In Tranquility, CA ten or twenty years ago. More that one hundred "historic" vehicles were there including this C-30 and a 1939 Ford Army Ambulance I purchased. Sales like that don't come around very often.

CB, number one parasitic DRAW on anything we work on is the integral regulator inside the alternator. We've replaced so many of those (entire alt assembly) for killing batteries. A heat gun picks it off pretty quick. On a unstarted vehicle the regulator/connector is significantly hotter than everything else under the hood. Sometimes you can feel it with your fingers.
We check draws with a low amp inductive clamp connected to a Snap On Vantage.