Replacement alternator

That looks like a powerful alternator, with a real thick output cable. How do you know it charges poorly at low rpm? Have you measured the current output with a clamp-on DC current probe or an in-line ammeter? The field connector looks similar to the alternators on my M-B 300D's. Those have an internal Vreg module. Seems like I have seen similar connectors in other posts about "best alternator to use", maybe from modern VW's.