This is the only episode in which during the opening credits Number Two identifies himself as 'I am Number Two' instead of 'The new Number Two', which strongly suggests that this episode is an immediate sequel to 'The General', especially since Colin Gordon plays Number Two in both episodes.
The spy who is unmasked by Number 6 near the end is voiced by Robert Rietty
The actress who played 'B" died a week after this episode was aired.
No 14 only has one dialogue scene with No 6 in "A. B. and C." despite their frequent interaction.