The exterior scenes outside of what is supposed to be the Hagar's European mansion were filmed at 380 South San Rafael Ave, Pasadena, California - the same house used as the exterior of Wayne Manor in the "Batman" series that was also on the air at this time.
The briefing scene (Phelps entering the industrial shop, turning off the blowtorch and listening to the tape) is re-used in The Phoenix (1968). The only changes are the audio content of the tape and the photographs of the Hagars.
In the final pursuit, both cars are shown driving on the left side of the road, even though both Italy and France are right-hand rule countries (and were in the 1960s). The cars themselves, as well as the truck Barney drives, are configured for right-hand rule driving, with the steering wheel on the left (although Phelps' car has a Mercedes-Benz logo on the trunk, its speedometer is calibrated in mph, not kph).
The dossier scene is re-used footage from The Widow (1967). The dossier for Dr. Premel, the "special guest agent" from that episode, can be seen on the top of the agents picked for this mission.
This is the first time one of the IMF's adversaries is female.