What does the Millennium Crown have on the base Ian is it named with a limited edition number and dated 2000?
Seems poor marketing to issue a Millennium Cascade paperweight to celebrate the Millennium in 2000 without naming it on the base and signing it 1999?
I have never held a Millennium Crown in hands and consequently I don't know how they are signed - but usually limited editions are signed with name, edition number/edition size. The year as signed by Selkirk is always the year a particular weight was made, irrespective of design or first issue - which is why you can find the same weight with different years in many cases. Whether the Millennium weights were still made after 2000 I wouldn't know - they don't show up in later catalogues.
With Millennium weights there might be a dilemma: many of them will have been produced in 1999 - to be in the shops in time. I would still expect them to be signed with name and edition info - if this is missing (as in your weight) this might indicate a design study, not one of the final, limited edition production weights.
Several clear glass weights are not inscribed on the base, as this would influence the appearance - I have a Bluebell weight for example, where this is the case, but it includes a PH cane instead; name and edition info are on the certificate only.