On the old reg / brush holder, the brush nearest the black regulator cap is worn beyond contact and would cause a no charge condition.

There is a resistor on the old one that runs across the regulator portion. Near as I can tell this was done to provide some charging even if the regulator failed. The resistor value would key off of electrical requirements that never go away if the car is running. ( Electric fuel pump , engine computer , injectors , ignition system , transmission control. )

Not all cars use this resistor, my 97 SL 320 has one but the same regulator used on a Audi or SAAB does not.