The Stamp by Brian A. Hawkins

A single strange postage stamp – hidden for decades – surfaces in Paris, threatening to expose a shameful secret of World War Two that could have changed history.
On a holiday in Europe, New York ad man Ted Oakley and his girl Jennifer Swanner become innocently entangled in a web of international intrigue when they unknowingly buy a vintage object concealing the stamp.
The work of a master Czechoslovakian engraver in 1940, this solitary stamp is sought after by an unscrupulous Zurich rare stamp dealer, The Keeper of the Royal Philatelic Collection, the Deputy Private Secretary to the Queen, a powerful Japanese millionaire and MI-5. Each have their own private reasons and will go to any lengths to obtain it from an unsuspecting Ted Oakley.
In Paris, Zurich, London and Tokyo, their frantic search for this philatelic time bomb involves the rarified world of big-league stamp collecting, the inner workings of an exclusive London Gentleman’s Club, a stylish Swiss sapphic nightclub, and Germany’s surreal Karl May Society.
In their pursuit of Ted Oakley and the stamp, three people will be murdered, and the final fate of this tiny paper rectangle takes a turn that none would have expected.