I still haven't been back to investigate. [Linked Image]

I checked from hot to ground (metallic oulet box) at the 6-50R welding receptacle and got ~140V. Same on each leg. I confirmed it was fed from a single-phase panel. I got ~120V to ground (panel chassis) on each leg at the panel itself. [Linked Image]

The walls are finished in between the outlet and panel. Judging by the connector in the box, it is piped with EMT - and there's no bond wire in the pipe.

Supposing the EMT is not providing a bond path anymore - perhaps through mechanical damage or dissolved in the slab - isn't that the same as turning your 4-wire delta back into a 3-wire (ungrounded) delta? And isn't ~140V (240/(root 3) what you should read from phase-to-ground on an ungrounded delta system?