I seriously doubt what John has is an open Delta setup.

Nothing mysterious here; it's just a corner grounded Delta setup, as I previously mentioned.

Here's some drawings of different configurations of Delta setups and how they can be grounded. I vote for drawing B in John's case. A to ground (or C phase) 460V, B to ground (or C phase) 460V, C to ground 0V, A to B 460V

Drawing C is a 4-wire Delta. A to ground is 120V, C to ground is 120V, B to ground is 208V, any phase to another is 230V, and of course all three are 3-phase 230V