Check for a corresponding kink under the top of the downtube, it's probably had a head-on prang.
That’s the first thing I’m going to look for when I see it in person. It’s also the first question I asked the owner when he was describing the situation. He claims no other indication of damage anywhere on the frame.
@Five-Hats started
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Check for a corresponding kink under the top of the downtube, it's probably had a head-on prang.