Thank you for all your comments so far.
As some were clever enough
to find the ebay auction - let me explain in a bit more detail. When I came across the auction during my regular search for Scottish paperweights I felt this was likely not a Caithness weight (not only that I couldn't find it in my database, the signature including a year would be quite unusual for Caithness). So I contacted the seller about the attribution and got as response that this was mostly guessing - BUT - the seller showed interest in a correct identification: this has to be commended, as it is not always the case (right now another seller is insisting that his Perthshire PP1 is Victorian - stating in the response that several other people have pointed out this mis-identification before!).
I recommended to ask here - and offered as alternative to post the question, if permission to use the images was given (to have the images in the post after deletion on ebay) - which was given.
Now back to the weight: as others I assumed the "1999" to indicate the year of manufacture. Whilst Caithness has produced several fairly "modern" designs more recently, I don't think the weight has a "Caithness 1999 look" - but might be wrong, of course.
Further comments are therefore most welcome!