Alternator with a built in voltage regulator?

Appears so, but those can be a problem for a couple of reasons. The voltage sensing is right there at the alternator, so typical voltage drop in the factory undersized harness can cause the battery and system to see under voltage

If the alternator is much more than 45A, it can damage or further damage old wiring. The big culprits are terminals and connectors inline from the alternator, through the ammeter, the bulkhead connector. And the factory wire size is too small