Hi @c11,
I only have one book on my shelf exploring Welsh traditions (The Book of Druidry: A Complete Introduction to the Magic & Wisdom of the Celtic Mysteries by Kristoffer Hughes) - I took a peek and it doesn’t look like it mentions Coelbren or Tellstones. But I suppose that isn’t surprising, as when I run a Google search for “Tellstones”, the only result is the book by Adam Bryn that says:
“Tellstones may be one of the least known (and certainly least documented) methods of divination to come down to us through the ages.”
That does seem to be the case!
But there are more results for a search on the “Coelbren Alphabet” aka the “Bardic Alphabet”. It looks like there are a few books and resources out there in addition to Bryn’s work.
I read the Wikipedia summary and, while one can never be too sure about Wikipedia as a source, it looks like there may be some 18th century drama involved 
The Coelbren y Beirdd (lit. ‘Bards’ lot’) is a script created in the late eighteenth century by the Welsh antiquarian and literary forger Edward Williams, best known as Iolo Morganwg.[1] From Wikipedia: Coelbren y Beirdd
It looks like Edward/Iolo had a tough time persuading people that his Bardic script was “authentic” (although it’s unclear what exactly they mean by authentic - ancient? Used on a large scale by many druids? Not something Edward/Iolo made up? Something else?). His son also worked to try to persuade others about the authenticity of the script. It looks like the script was never “proven” and that some scholars believe it to be a “hoax”.
Iolo Morganwg suggested they were originally the work of bards from Glamorgan who had their manuscripts copied into collections stored at Plas y Fan, Neath Abbey, Margam Abbey and Raglan Library, and compiled by Meurig Dafydd and Lewys Morgannwg, amongst others, in the 1700s. These were suggested to have again been transcribed by Edward Dafydd, John Bradford and Llywelyn Siôn. Morganwg suggested that he had collected some of Siôn and Bradford’s manuscripts, while the majority, including all of Lewys Morgannwg’s sources, were lost. Numerous scholars, such as Glyn Cothi Lewis, concluded that the script was a hoax.[5][6][7][8] From Wikipedia: Coelbren y Beirdd
Of course, a handful of scholars saying something is a “hoax” does not necessarily mean they are correct, nor would it invalidate a form of divination that people are using and enjoying. Similar to the controversary around the Celtic Tree Calendar (some claiming it is “ancient” when it is actually a modern interpretation), whether or not something is “ancient” or “modern” is not nearly as important (to me) as whether or not the person using it finds it meaningful.
It’s helpful to know lore and history to make educated choices within one’s practice, but ultimately, we each hold full power to decide which tools/methods/traditions work for us. Meaning can come from anywhere. All that is to say, I’d encourage you to keep studying the Bardic Alphabet if it calls to you! I’d love to hear what research and wisdom you find.
Blessed studies, c11! 