In the opening scene, the U.N.C.L.E. Special Mark II makes its only appearance as a carbine with shoulder strap -- a very logical and necessary addition.
In the data room at U.N.C.L.E. HQ, Waverly kids Solo, giving us a hint that their relationship is slightly more than superior/operative, almost father/son. (Ha! You thought I was going to write "uncle/nephew," didn't you?) More important, when the young agent's voice tape cuts off, Solo and Waverly share a moment of silence for a fallen comrade.
Where in the Andes is Barridqua supposed to be? When I first saw the show, I presumed it was in Peru, but Chile also has resort areas wedged between the Pacific and the Andes. (Santiago, Chile, has a climate much like Southern California's, so having mountain/desert areas of CA stand in for South America works quite well here. In the mountain scenes, the clear hard skies give you the distinct feeling you are indeed at 10- or 12,000 feet.)
However, the aerial map Solo and Waverly inspect has water to its right, as though it were on the east coast of South America. Of course the aerial shot could be of an inland lake or bay -– such as the Golfo de Ancud, a little south of Santiago, and the Golfo de Guayquil, in Ecuador -- but then Solo would refer to the "shore" rather than the "coast."
The famous party scene has been discussed to the point of Extreme Unction. I’ll merely add that Illya's telling Marion about Ravel and Bufferton (for the audience members who haven't seen "Quadripartite") is sorta clumsy. After all, she knows who they are: their partners almost killed her only a few months back.
This story marks the debut of the Illya we have come to admire and love: the cool, job-focused master of disguise, clearly Solo's equal partner (though the "Machiavellian" plot is all Solo's; Illya says so). Sparks fly in his exchanges with Marion as he tries to enlist her aid, and he has one memorable line. (Marion: "Will we see you [in Barridqua]?" Illya: "Well, I'll see you there.")
I like this Solo/Illya relationship better than the sometimes too edgy, impatient banter we get in later seasons. Here they seem to admire each other's abilities, and we have none of those "I should have known it was you; you were stumbling over your own feet" and "Your smoke signals were almost as illegible as your handwriting" jabs that they toss at each other later.
The carbine Bufferton uses is a "broomhandle" Mauser with shoulder stock -- a gun that might have helped to inspire the attachments for the Special guns. Lt. Manuera has one at the climax, too.
Bill Koenig, on his Episode Guide, has pointed out more than once the central irony of this story, that Bufferton says he will know that Gervaise loves him when she weeps at his grave . . . and then actually hears her weeping over him as he dies from Illya's bullet. This, Bufferton's love for Gervaise and her grief for him, makes them both more comprehensible (though not sympathetic), and gives them a human dimension that borders on tragedy. Antony and Cleopatra, indeed.
What military force is it that backs them up at the climax -- Federal troops of the country they're in? I don't think we ever heard.
A wonderful tag, with Solo reaching for one "for the road."
Verdict: a fine sequel to "Quadripartite." With its witty dialogue and clever tricky plot, it's a good one to show someone who's curious about MfU, but not yet a fan.

Marion: "Two words: Balder Dash!"
Marion (to Solo): "You're nothing but a whirling mass of plots and schemes!"
Bufferton: "Your assurance borders on the arrogant."
Solo: "You know, I used to worry a lot about that, until I realized it only offended people such as yourself."