I've been neglecting this list and never saw this thread until today. Glad you finally found a grill on cover. Presumably it originated there, but given the condition and the fact it is not tied means that down the road someone is going to have trouble proving it was there at the time of cancellation. Anyway, now you have a sense of how well they do (or really don't) show up. But you raised another question that I think is interesting:
was the postal service really stringent about using the appropriate stamp at/of the appropriate issue time? Did people just not use an "old" stamp if they had one?
Back in the 1870s and 80s, most people went to the Post Office to pick up their mail. When they wanted to mail something, they also took it to the Post Office and had the clerk determine the postage, apply it, and send it off. In those early days only a portion of the people using the mails bought and held stamps for postage. No coils, no booklets, no vending machines - you went to the clerk. If you did buy a sheet to take home and use there, you had to keep the stamps from sticking to each other or to the furniture. Reliable gum was a frequent matter of complaint and concern to the stamp producers even among the postal people.
In a world like that, where stamps were most often applied in Post Offices, letters received the latest issues in production unless the clerk had old stock to use up. For these and probably other reasons, there seems to be a high correlation/proximity between date of production and date of use.
Something else to note is that during the decade of the 1870s not too many cancellations bore a year date even in the townmark. So it can be difficult to tell when a letter actually passed through the mails, and so also when the stamp was used.
Welcome to the world of the Bank Note Issues. If you like puzzles, it's a great place to be.