I'm not about to turn my entire collection upside down, but I did quickly check a small sample.

Seven out of 13 had a HKDNP stamp. What they had in common was that the year was either 2007 or 2008. None of the other houses are from those two years. Could be a complete coincidence, of course.

One of the HKDNP houses is one of those empty ones handed out on certain ME flights, which begs the question what duty was not paid.

I also have houses without a year stamp, but I'm pretty sure they are all (decades) old. Think Rynbende.

As for the value of these houses, that's in the eye of the beholder. For most of the world's population it's probably close to nil, at least once they've imbibed the contents. I suspect that only the most hardcore collectors give a hoot about what it says on the bottom, the one side nobody ever looks at, and that has zero esthetic appeal.

Johan