The OEM closed field (round back) alternators were iffy. If sufficientt ground was found through mounting hardware a actual ground wire wasn't needed but recommended.
Most of todays replacement alternators are the isolated field (square back) type. That would need one of the 2 field terminals grounded for it to function with the early type regulator. Those early round back units didn't put out enough juice to operate todays headlights and other accessories we desire anyway.