There are still more series central stations out there in various places - mostly in the midwest... I was in one about 15 years ago in Wilmette, IL but they've decommissioned their series since then. But these are far and away the oldest regulators I've seen! We figure they must be from the 20s.

The Sioux Falls municipal once had a set in their plant, but that has long since been scrapped (the only equipment left in their old light plant are the engines / generators all of which are too big to move).

Another town I recently discovered once had a series system is Belle Fourche, SD.