A book of second class stamps was found in a victim's pocket. Two-tier postal rates (first and second class stamps) weren't introduced until September 1968.
Jean uses the word "leverage" (as in "what leverage has she got over you") and pronounces it the American way, "levverage", rather than the British way, "leeverage". The American pronunciation was virtually unknown in Britain in the 1950s.
If John and Alice had a baby in 1943, then ten years later, when most of this episode takes place, she would be seven years old, not seventeen.