On the 1939 Register his dob is 9 Feb 1879:
https://www.ancestry.co.uk/interacti...nSearchResults
David James B Jones married Charlotte Gwendoline E Thomas Jul-Sep 1905 Holyhead registration district. So Charlotte is the same person as Gwendolen.
There are two David James Jones births registered Jan-Mar 1879 Newcastle in Emlyn district, which included part of Carmarthenshire. Both with MMN Jones, which doesn't help. I was hoping one would have MMN Basil! I suppose we should be able to trace him back via the details of his father though.