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For more than 40 years, amateurs and pros alike have put forward dozens of theories, many quite plausible and backed with some evidence. But the story seems destined to remain a delicious historical mystery. (Part 4 of a 4-part series)(PDX, Multnomah County; 1971) (For text and pictures, see http://offbeatoregon.com/1306d-db-cooper-part4-the-theories.html)