There is a set of 4 (or 5) script style RF overprints on US postal stationery. They are all fakes and believed to be the work of one Mr. Stolow of the Economist Stamp Company. They go for about $10 each as curiosities (or fakes, depending on your view point). See the article "Counterfeits, fakes, forgeries & other oddities of U.S. Airmail Postal Stationery" in Airpost Journal vol. 79, # 8 and the article U.S. Envelope Fake RF Overprints in the journal Postal Stationery whole # 373.
Scott catalog editors and collectors had a terrible time determining genuine RF overprinted items until the seminole work of Henry Goodkind, summarized in the booklet United States RF Overprints on Air Mail Stamps and Stationery (1944-45) published by the Collectors' Club of NY in 1958.