Just back from Holland!
As you know, I have quite a large collection, including the large bowl in ruby. I have also been following that other one on eBay, but the price is triple what I would be willing to pay. However, mine measures 13¾ (35cm) across, so quite a beast, but does not have a flat base. The amethyst colour I have seen and yours looks a little like Flair, but I'm also not sure.
One thing I do know is that there are some oddities in the Flair range. I am in touch occasionally with Peter Robinson who worked at Webb during the 1960s and he even has some of the pieces that didn't quite make it, or were experimental, or "mutants" as Peter calls them!

I'll see if he wants to join in as I believe he's been on GMB before. It could be that yours is a mutant?