LOL. Could be a shocking revelationThe neutral support wire you describe is a neutral, not a ground. If they didn't need the neutral it wasn't connected.
If the 480V service is supplying loads that do not require a neutral, say an MCC, why would one be installed? But normally you would still bond it to ground.
Back to the buried ground. Yes that's not the way it would be done today, but back in the day who knows what they did? There may be an entire ground grid installed at that location. Tell you what, instead of digging up the ground next time you're there, how about you string out a piece of wire between the utility and the buildings's ground and take a measurement. I don't have to tell you to check for voltage first.