Well, that would explain the leaky faucet in the bathtub, even after replacing the conical washer. Normally this piece would be replaced if it exhibits this kind of damage, but at 1:00 o’clock in the morning, there aren’t a lot of plumbing shops open to buy a replacement. Also, why throw away perfectly good money for something that can be repaired for free?! Put the seal in the mini lathe, faced it off until the surface craters were gone, and put a nice chamfer on the ID for the washer to seat against. Reinstalled with a 5/16″ allen wrench, reassembled the valve, and it holds water. Success.