Well, they're not completely gone.But useless, nonetheless.
I have no idea what the original thread was. I bought it used knowing the threads were hosed. Previously, it had an M14x1.5 "oversize" plug. I'm hoping there's enough material left in the hole for a standard M14x1.5 HeliCoil. My local hardware actually has generic drain plugs in stock, and a pretty good assortment of sealing washers. The plug I got was the perfect size for one of the Neo magnets I have floating around the garage.
There was some pretty bad machining chatter. Bad enough that I hit it with some emery cloth. Should be good now.