Yes you are correct it is a voltage regulator. You will need to determine whether the alternator fitted has an inbuilt regulator or not (it may have been changed at some point and thats why regulator was not fitted).

If not and you want to replace it with a in-built regulator model this can be done. Refer to the tech wiki for detailed information (or I know in the Datsun 1200 Service manual it shows wiring variations for with and without inbuilt regs.)