THE investigation

Capital S.

Contrex, do you mean this?:

Agatha Christie The Secret Adversary: Just before the Bond Street Tube station they crossed the road

At that time, was THE optional or would the existence of THE convey a different meaning?

At the time 'the' was usual, now it is rare and archaic. The use of 'the' would not convey a different meaning.

Thank you~
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What's the difference between using THE and not?

eg When is (the) course selection time./(The) course selection time this year will be on 8th August.

My theory:
If 'course selection time' is regular every year, at the same time, then THE must be omited, like 'I have lunch at noon', not THE lunch; if not regular, THE must be used. (After typing this, I feel strange, because in English I've heard 'This is payback time', not THE payback, etc., though it is irregular)