That was my first thought as well. But the wear or shape is over such a long segment of the threading. If it was just from a keyed washer held by a loose headset I'd expect it to be a far more narrow side to side damage channel with a more or less normal looking key groove above and below. I suspect in this case the "key" is more of a flat segment or a semi round segment and that most of the groove is as it came from the maker. But that sure doesn't explain the segment of U groove that extends down onto the head tube unless the fork and headtube were cut together. Hmmmmmm... seems like this'll be one of life's great mysteries.

But that obvious rip in the threading and hole you can see light through is the kicker. Now THAT seems like the keyed washer could have caused that damage. There even appears to be some form of stretch damage that finally snapped or wore through where the hole is located. If I'm right then no wonder the headset is feeling a little coggy and less than fresh thanks to the angle the fork is holding the uppper bearing.