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There was a jet airliner flying overhead, which Towns estimated to be flying at 35,000 feet. Undoubtedly a Boeing 707, or other such aircraft from the '60s.

Very early in the film a copy of Playboy magazine is seen being read.. presumably a recent issue.. date it from that.

I would like to know the answer to this one, too.
"It was to be announced at the Party Congress on Monday. As you know, the Premier loves surprises."

There is also the conversation between Ian Bannen and Hardy Kruger about WWII, and Dorfman not being involved in the war, I assume because he would have been too young.
Bannen, as Ratbags, jokes that the war might have turned out differently if Dorfman was around for the Germans.
"Congratulations, Major. It appears that at last you have found yourself a real war." Ben Tyreen

Ahyes. Plenty of clues. Cobb's transistor radio. The ol' flying boxcar design aircraft. Hunting for oil in Libya.
Has to be the early 1960s. Before Colonel Qaddafe took over and kicked out all the Americans.