Are there suddenly dozens more genuine Rembrandts in the world?

There are if art authorities accept the findings of Ernst van de Wetering, the Dutch art historian and longtime head of the Netherlands-based Rembrandt Research Project. In its sixth and final volume, published Wednesday, Mr. van de Wetering reattributes 70 paintings—often discounted by previous scholars as well as the institutions that own them—to the Dutch master. They include four at New York’s Metropolitan Museum of Art. Of those, Mr. van de Wetering is quick to emphasize “Portrait of a Man,” also known as “The Auctioneer,” dated 1658 by the researcher.