Here David is in 1861 and 1871
Lletynest 1861
David Davies,Head,Widow,73,Farm Baliff,Llanpumpsaint
Margaret Thomas,Unm,Servant,27,Llanllawddog

Llettynest 1871
David Davies,Head,84,Farmer 11 acres,Blancoso?Llanpumpsaint
Carms
Margaret Davies,Wife,36,Llanpumpsaint
Ann Davies,Dau,11,Llanllawddog
Richard Davies,7m,Son,Llanllawddog

Looks to me as they are the same name right age in both censuses that David married his servant.He was obviously enjoying his old age and still producing children.
As hes called another son Richard think we can assume the first Richard died.
Another thing to look into is a will looks to me like David owned the Farm he was on for 30 years or so.So would assume theres a will.
Also be interesting to find out whos living on the Farm in 1881 assume David has passed by then.Margaret young enough to remarry does he leave it to her or his children?.
If you have no joy ill do a look up in Llanllawddog Parish Records next time i go see what infos there problem we have is that we dont know if all the earlier children were born in Llanllawddog?.
And a better chance now is to get the marriage cert of David Davies to Margaret possibly Thomas .As this will give you Davids Fathers name.
Good Luck Karen