How I can tell if a Saxbo vase is unique?  

I collect ceramics from Saxbo, and as there are yet to be any official books or other sources of information about Saxbo, some questions are hard to get answers to.
I often see shops and private sellers describing their Saxbo items for sale as being unique. This may of course be wishful thinking, but some of them might actually be right, so how can they tell? It must be something in the marking beside Saxbo's logo and the artist's signature. Sometimes there are 3 or 4 small dots and othertimes a small cross.
Any advise, theories or opinions about Saxbo are more than welcome 🙂