Here's a color/black and white advertisement for a Spy-Pen based on The Man from U.N.C.L.E., found in the October 1966 issue of Marvel's The Avengers comic book (specifically, issue #35). The advertisement features Robert Vaughn, David McCallum and Leo G. Carroll and offers buyers the ability to "see" through walls. It was "developed by the German Secret Service in World War II" and is "Used By Interpol." The pen is a "precision optical instrument" with "three precision ground lenses, not just two like some imitations." Apparently, it is so clear that photographs can be taken through it. And it only cost $1.98 plus 27 cents shipping and handling.
Advertisement for Man from U.N.C.L.E. Spy Pen
Copyright © Vista Publications, Inc., 1966 
Does anyone still have their Spy-Pen?
1 From The Avengers #35, December 1966, Page 6.