At Hoover Dam instead, first time I recall real machinery, in a rather inconceivable place, astounded. Dynamos, overhead crane, lots of space...also a very large swing, long bed horizontal lathe nearby, probably Axelson, Monarch or Lodge & Shipley, possibly American Tool Works. I've tried googling it before.
Made quite an impression on a 7 year old...60 years later, still look back on it.