A casual lunchtime snap, or the world's most iconic publicity stunt? This photograph of construction workers casually eating their lunch on a skyscraper beam suspended high about Manhattan can lay claim to being one of the 20th century's most recognisable images. Yet, in the run up to its 80th anniversary today it has emerged that, far from catching the subjects unaware, the image was set up as a publicity shot for the Rockefeller Center. The identity of the photographer of Lunch Atop a Skyscraper is unknown. He or she was among a pack of snappers sent by news agencies to cover the event at the RCA Building. Another, less celebrated, image shows the workers pretending to be asleep on the beam. Ken Johnston, chief historian and archivist for Corbis Images, which owns the rights to the photo, said: "The image was a publicity effort by the Rockefeller Center. It seems pretty clear they were real workers, but the event was organised with a number of photographers. The photograph was taken on 20 September 1932, during the construction of the RCA site - later renamed the GE Building - which forms part of the Rockefeller Center. The original caption on the photo marked that it would be the largest office building in New York City, the archivist said. Mr Johnston said Lunch Atop a Skyscraper was probably "the most recognisable" of Corbis' 20 million-strong catalogue and is its "biggest selling historical image." It is licensed more than the images Corbis owns of Albert Einstein and Martin Luther King. For a decade, it was believed that Charles Ebbets was the photographer after his son presented compelling evidence he was at the site that day. But after it emerged that there was more than one photographer present, Corbis is no longer sure he took it. Huge efforts have been made to identify the construction workers. About 12 years ago, Corbis hired private investigators and enlisted a New York newspaper to help find out who they were. "For most of the men on the beam we got multiple names and for some no names at all," Mr Johnston said. Because it was so mixed up and impossible to cross-reference we thought it could not be solved.