Dale and Phyllis bought their Boss 429 Mustang new on March 18, 1969, from a dealer in Memphis, and then did something almost nobody does: they kept every piece of paper that came with it. Nearly five decades and 21,400 miles later, the car crossed the block at Barrett-Jackson in Las Vegas still wearing its original drivetrain and carrying its window sticker. Read on for why that documentation is worth as much as the engine.