There is a Benjamin ( farmer). with wife Sarah in the 1881 census- and son John all in Llanboidy...could they be yours?
There is also a David ( aged 38, b CMN )with family in Aberdare GLA - he is a cooper
I amnot sure if you have discounted these or not, so let us know....if they are yours, then someone will probably find them in the earlier censuses for you!
Cheers
Jocelyn