When her husband headed to a taping of "Antiques Roadshow" with some old books to be valued, a Welsh woman stuffed a plate from her "rickety" sideboard into a shopping bag to bring along. To Mrs. Jones's surprise, an expert gave the plate a value of £100,000, a record price for a plate in the 30-year-old show. Turns out to be an 18th-c. piece commissioned by the Prussian East India company for Frederick II, and decorated with the arms of the Hohenzollern family, the order of the black eagle and Maltese Cross...