As we don’t want another flood, the Boy and I spent more than an hour getting at the offending floor drain today. Naturally, it is under a stud (supporting an electrical outlet), and so, it wasn’t a simple matter to get at it.
The hole is simply straight through the concrete, 3 1/2 inches in diameter. Rona and Home Depot were no help whatsoever in plugging it up. I’m hoping a real plumbing supply may be able to propose something on Monday.