This is a parody of The Maltese Falcon. Stratford Johns is playing the part played by Sydney Greenstreet and Peter Lorre's part is being played by Ronald Lacy.
This is one of the few episodes in which Steed uses a gun.
Tuttle Lemkow and Ronald Lacey both appeared in the first Indiana Jones film.
Just a year before this episode was filmed, actor Peter Swanwick worked on several episodes of The Prisoner with director Don Chaffey. Unfortunately, this was one of Swanwick's final productions: he died in 1968 at age 46.
Towards the end, Steed and Tara share the pearl infused wine, "the world's most expensive drink". Reputedly, Cleopatra impressed Mark Antony by crushing a large pearl in wine to make a drink worth a king's ransom. Small problem, any wine acidic enough to dissolve a pearl's calcium carbonate would be vinegar. (Although back then Roman soldiers did mix vinegar and water as a beverage.)