Hello - does anyone have a clear explanation as to why there are 3 (or more perhaps) different types of markings on KLM houses that are not vintage? On the plane, you are currently given a house with a date and number etched on the bottom and the usual KLM Bols stamp. However, there are those that have the date and number but are also stamped "HKDNP" - Hong Kong Duty Not Paid. Those seem to vary occasionally in small size differences to some of the others and are most likely specially made to include the mark that is glazed over. Why are there so many of those out there? Does KLM try to get as many out through HK to save money and then distribute elsewhere, are they even given out by KLM outside of Asia? It cannot all be due to Hong Kong connections via KLM given the quantity, can it?
There is also a 3rd kind with the house number etched on the bottom but no date. I have heard people refer to them as "duplicates" but I have no idea how or why they came about or how they are distributed. Are any of HKDNP or no dates one fakes or was there a reason for this? If not fakes, I assume the dated ones are more valuable (maybe that is not the case) but I don't know what the effect of having HKDNP has on value.

It has proven hard to find answers to these but someone must know!

Thanks!