On one side of the collector’s copper plate was a simple, antique landscape. On the other side, a British expert gleefully discovered, was an etching by Rembrandt, signed and dated 1656. The etching tells the story from Genesis about the angels God sent to the aged Abraham to tell him that he would have a son by his wife Sarah. It shows three angels sitting with Abraham. Behind them, Abraham’s son Ishmael plays with a bow and arrow. Sarah peers from the darkness of a doorway. The plate will go on show at Christie’s in November.