Hi fogo4, thanks for stopping by.
This claim about the Flat Earth Society seems to mostly appear in travel guides, advertisements, and parody sites. It appears to have very little to do with the Flat Earth Society or its past incarnations. As it stands, the FES does not believe that the world has corners, and there are no known records of it ever believing that.
That said, I would be interested in finding out how this rumour originated. There appears to be a link between this claim and the
Flat Earth Society of Canada (a terrible misnomer, since they weren't actually a Flat Earth group), which was most likely originated by John Robert Colombo in his book
Canadian Literary Landmarks (ISBN 978-0888820730). Colombo never had any history in the Society, and has not documented any of his encounters with it either, so it is most likely a work of his vivid imagination.
Sorry for the sparse information, but that's all I know. Hope it helps anyway!