Works by Rembrandt, Raphael and Van Dyck are being offered for record prices in London this week at Old Master auctions that may raise as much as 81.1 million pounds ($133 million). Among a diminishing supply of great Old Masters on the market is a previously unrecorded portrait by Peter Paul Rubens (low estimate: 4 million pounds), at Sotheby's. Barbara Piasecka Johnson, the Johnson & Johnson heiress, is selling Rembrandt’s 1658 “Portrait of a Man With Arms Akimbo” at Christie’s...