Who inherited the folded-up birth certificate? Were they descended from one of the daughters?
It was the daughter of Gwynneth who had it. Gwynneth married Morgan Bevan and these are my husband's grandparents.
Here is a list of William Ivor Richards and Sarah Ann Harris' children:
John Richards(1910–)
Lilian Evelyn Gwendoline Richards(1910–)
1911 census - Abersychan, Monmouthshire, Wales
Flora Olive Helena May Richards(1912–1922)
Gwynneth Phyllis Eileen Richards Mrs(1914–1956)
William John Richards(1917–)
Ismay Beryl Richards(1919–)
Eric Richards(1923–)
So maybe the 1930 certificate is Ismay's as she was the last daughter or maybe it is Gwynneth's and she needed a copy of her original for something??