It turns out that the Minimal and Maximal range were designed by IOW but outsourced to Isle of Man glass. IOM glass was staffed by ex Wedgwood glassmakers and apart from those made for IOW, they also made more than they were supposed to and sold them to a company called Espagna International. Needless to say, both of those naughty companies are out of business.
So these on eBay can be regarded as fraudulent copies. Which is an interesting status. The seller is not breaking any rules and they are effectively second-hand fraudulent copies. Apart from any IOW marking, these would be indistinguishable from the originals.
Does anyone have an IOW original, can they confirm if marked?