It was in the winter of 2016 that Jan Six, a Dutch art dealer based in Amsterdam, made a discovery that would upend his life. He was leafing through a Christie’s catalog when he spotted a painting featuring a young man wearing a dazed look, a lace collar and a proto-Led Zeppelin coif. Christie’s had labeled it a painting by one of Rembrandt’s followers, but Mr. Six knew it was by the Dutch master himself. Today on The Sunday Read, a look at Mr. Six’s discovery of the first new Rembrandt painting in over four decades, and the fallout from finding it.