Revealing: Our heroes arrive at an RAF base. Purdey hears a loud aircraft noise and looks up to the sky - cut to shot of a fighter jet. For a few frames after the cut, a caption that reads "The Engineer in the RAF" is just visible. It would seem they 'borrowed' some material from an RAF training film. (Indeed, a later shot of shells exploding was obviously taken from a videotaped TV programme, judging by the 'lag' effect caused by the tube camera that would've been used).
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