Late to the game. . . again. That's me. Am I correct in assuming this is a "round column" mill? When a tube is "struck" by some means, it will tend to spread in a "flat" dimension. Side to side. . . It may, or may not show a dent, depending on how hard it was struck. Just knocked a little out of round. Dropping the column, even in a soft material like sand, can possibly cause the "out of round" condition. I don't know an easy way to find where. You're off to a good start with marking fluid. Swinging side to side may make it more obvious.
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