That's how I read the letter: a reservation has been made, but the ticket specifically mentions the additional 7s 6d charge that must be paid either with the fare or on departure. That wording suggests that the fare has not yet been paid.

From hazy memory, I think that the phrase "passenger ticket and baggage check" may derive from the Chicago Convention. This would mean that the concept had been around since long before that BOAC letter, which does not use the phrase. (Incidentally, I wonder when the phrase fell out of use. Was it with the advent of e-tickets?)

But I agree with subject2load: that letter takes star billing, as I think it's less common to see one of those than a "passenger ticket and baggage check" of the same era, and it's such an evocative document.