Having said all of the above, does anyone care to explain this one? It's one of the few first day cancelled items I have that has been postally used thereafter:

An anomaly? Or due to a lazy or unalert postal clerk that let it through? Perhaps. But I have 3 or 4 of them done the same way.
Right or wrong, I do seem to recall that someone had told me years ago that these specific first day cancelled postal cards did have postal validity after the first day of issue as the first day cancel did not specifically indicate a city/state and were basically CTO's provided overseas at the Brussels Philatelic Exhibition and sold in the US through the former "Philatelic Sales Division" and therefore did continue to carry its US postage value when used in the United States.
I was never able to confirm or deny that statement, but it would appear to be a plausable explanation given that the card was used a year and a half after the first day cancel was applied.