Originally, it grounded to the voltage regulator. The early regulators were insulator mounted which required the grounding connection to the generator. The link illustrates an old style regulator with the ground terminal on the left side. Rubber grommets with metal inner bushings were fitted to the mounting lugs for firewall attachment. They are missing from this example.
https://www.ebay.com/itm/402327949163
A lot of the 39 models still had tree brush generators but it depends largely on whether it's a standard or a deluxe model in 1939.