Hi
Couldn't find a Seth Jones in 1861, and any births for a Seth Jones in the area were either too early or too late, but did identify a Zeth Jones in the timeframe and area that you are looking at:-
Zeth Jones Year of Reg: 1861 Qtr:A -M- J Dist:Abergavenny County:Herefordshire, Monmouthshire
Vol: 11a - Pg: 122
Maybe a possibility?
Also a marriage possibilty of a William Jones and Margaret Parry - Year of Reg: 1843 Qtr: A-M-J District: Newport M (which according to Genuki will include the Parish of Mynyddyslwyn) County: Glam, Gwent, Monmouthshire Vol 26 Pg 125. I note that Margaret has given her birth to be in Mynyddyslwyn, the probability of her remaining and marrying in the area would be reasonably high, her first child David Jones was given as being born in 1846 in the area and have also found a Margaret Parry b 1820 in the 1841 census H0107 Piece 751 Folio 37 Page 4 living in the Civil Parish of Mynyddyslwyn.
Apologies, as I cannot be more conclusive than this, but thought I would post what I had found.
Regards
Z
