NEAR THE SANTA FE PLAZA, a narrow courtyard between two buildings on Palace Avenue holds ristras, Talavera pottery, and crepe paper flowers. Wander all the way back for a piece of what was once top-secret history. A nondescript plaque above a table of retablos reads, in part: “All the men and women who made the first atomic bomb passed through this portal to their secret mission at Los Alamos.” Robert Oppenheimer, director of the Manhattan Project, rented offices at 109 E. Palace Avenue in 1943 and hired the formidable Dorothy McKibbin to register every single scientist until the offices closed in 1963. “This is probably the most photographed place on my tour,” says Lynn Clark, a docent for the New Mexico History Museum’s Historic Downtown Walking Tours. Originally a 17th-century hacienda, the building bears little resemblance to its World War II purpose. Even the address numbers have changed. (The Chocolate + Cashmere shop now bears the 109 number.) But Bob Kapoun, owner of the Rainbow Man, says that doesn’t deter history seekers. “We get all kinds of people interested in it,” he says.