In Stamp-Collecting Month, Consider Pastime Edition of Friday, Sept. 26 To the Editor: A single stamp, the British Guiana one-cent magenta, sold in June 2014 at Sotheby’s for a record-setting $9.48 million. By weight, a mere .04 grams, it is considered the most valuable item in the world. The stamp was issued in 1856 in British Guiana (now Guyana), and is the only copy known to exist. Interestingly, it was found in 1873 by a Scottish schoolboy who sold it…