Thanks, Keith. What you and Greg reported about the lack of a maker's mark seems to be fairly standard for this pattern of decanter.
Stuck in a less than satisfactory limbo, I decided to try something different. So I contacted both the stylendesign website proprietor and Andy McConnell, author of The Decanter, for their views.
The stylendesign website proprietor kindly explained that it is a Webb attribution that caused them initial difficulty for the same reason. However matching glasses that occasionally come with the decanters were marked. Also a rare matching jug was marked. The final step appeared in Glass of the 20's and 30's, a Millers guide by Frankie Leibe with Jeanette Hayhurst, 1999. On page 11 is a similar amethyst decanter which is positively attributed to Thomas Webb (where I've seen it in a book, Greg).
Andy McConnell kindly explained that he had one or two matching tumblers and confirmed the attribution above.
That seems to wrap it up nicely, except for one more problem. Why didn't Leibe/Hayhurst explain how they achieved the attribution?
If you read the two pages entitled Optic Moulding, you will notice a mention of unmarked pieces in the opening paragraph along with how pieces are attributed in general, but not mentioning marked accompanying glasses or any other type of go-with. At the start of the description of the decanter on page 11 they mention matching glasses without explanation. I suggest two possibilities:-
It was so obvious (to them) that they just forgot! ... or, more likely,an unsympathetic editor deleted a few words when trying to fit the words and images into the space available — without checking with the authors that the outcome still made sense.
Grateful thanks to everyone who helped, and apologies to Anne for not making it clear earlier that this topic hadn't quite finished.
Any other relevant points?
Bernard C. 8)