George Hamilton was able to demand and receive $100,000.00 for this film. He was at this time considered a hot commodity due in part to the fact that he was dating the president's daughter.
Jeff's watch is a Rolex Oyster Perpetual Submariner. Expensive at the time, it could be worth $15,000 or more in 2020.
Although this movie is a West German co-production and much of the movie takes place in West Germany (primarily Munich with fewer scenes set in Garmisch), there is only one word spoken in German in the movie: dummkopf (fool).
Jeff uses the typical "spy camera" - a Minox B - but with the rarely-seen Minox B/C flashgun that uses AG1 bulbs.
The miniature breathing device that George Hamilton uses at about 19 minutes is copied from an identical device seen two years earlier in the James Bond film Opération Tonnerre (1965). It reappears in a later Bond film, Meurs un autre jour (2002).