Hi there,
That James Williams later marries Janes mother Rachel after John (Jane's father died in a crane accident at the mine) and they have a daughter called Amy.
All the Thomas' come from Nantyglo.  I can't find a suitable William James combo on the 1971 in Nantyglo though.  Although if that comes under Aberystruth (which I wasn't aware, my knowlegde of Wales is not great!) then I guess it is a definate possible 
I have located 9 possible combos in 1871 though, all in Wales, ages vary from born 1862 - 1866 to cater for the age difference from the 91 census (29) to the 1891 marriage cert (25), only 9!!!
William Williams, father James (works in tin plate works) , mother Ann abt 1865  Llanguith, Glamorgan, Wales Son  Llanelly, Carmarthenshire  or
William Williams, father James (ag lab), mother Anna abt 1865  Penboyr, Carmarthenshire, Wales Son  Penboyr, Carmarthenshire.
William Williams, father James (labourer), mother Ellen abt 1866  Llanbedr Goch, Anglesey, Wales Son  Llanbedr Goch, Anglesey 
William Williams, father James (ag lab), mother Hannah abt 1866  Llanstephan, Carmarthenshire, Wales Son  Llanstephan, Carmarthenshire 
William Williams, father James (puddler), mother Elizabeth abt 1864  Trevethin, Monmouthshire, Wales Son  Trevethin, Monmouthshire 
William Williams, father James (labourer, land 10 area), mother Jane (transcirbed as James) abt 1863  Llanwinio, Carmarthenshire, Wales Son  Tre Lech AR Bettws, Carmarthenshire 
William H Williams, father James (master builder), mother Elizabeth abt 1865  Monmouthshire, Wales Son  Monmouth, Monmouthshire 
William Williams, father James (joiner), mother Alice abt 1862  Saltney, Flintshire, Wales Son  Saltney, Flintshire 
William Williams, father James (bargeman), mother Anne abt 1862  Llangyfelach, Glamorgan, Wales Son  Llangyfelach, Glamorgan 
There are a handful without fathers, with aunts/uncles/grandparents though, I guess he could be one of those..... (knowing my luck he will be!)