Well done everyone ...
The accumulated evidence does seem to point to Alum Bay. And it also shows the confusion that can arise. Sue's first link to an eBay item shows a weight similar to Gary's, but with what appears to be the regular Isle of Wight impressed "Flame" logo - although not easy to see in the photo (which is often the case).
So far, I have not found any references or images, other than Gary's and Ray's, to the "AB" impressed mark - so my "American" idea seemed reasonable.
But I did cover myself with Gary by also suggesting "English studio". (Had this been an "easy" id, like Leni's Clichy weight she has recently bought - yes Leni, it really is a mid 19th century Clichy, I could have sorted it out in no time). At least, I can now impress (so to speak) people at the Paperweight Club if anyone asks about the "AB" logo :wink: