What a shame you couldn't find Glynrhysglych, but as you say, it is probably long gone. That is a really interesting area, and it has fascinated me since we moved to that area. When the Epynt was cleared for the opening of the army ranges, the communities were scattered thereafter, with some going to Brecon, others to Sennybridge and some leaving the area completely. It is strange to drive over the ranges today and see how little remains of those thriving communities.
Regards
GS