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There's a will online at the National Library of Wales of a William Powell, yeoman farmer of Devynnock. The will was written in 1784, proved 1794, the executrix being William's widow Elinor.
The will mentions various relatives, but not a daughter Margaret, or any children for that matter. Bequests go to
nephew Penry (?) Powell, under 18 at time will was written, son of William's brother Thomas
nephew William, under 18, son of deceased sister Margaret Jones
brother John Powell
sister Ester, wife of John Thomas
Anne Lloyd, under 21, daughter of John Lloyd, deceased, of Trallong
another daughter of the same John Lloyd, also under 21, called Ma???
Thomas, under 21, son of the same John Lloyd
nephew John, son of John Thomas
Watkin, son of brother-in-law Harry Lloyd
nephew Thomas, son of brother Thomas
All the bequests except one are to be paid only if Elinor isn't pregnant at the time of William's death.
http://cat.llgc.org.uk/cgi-bin/gw/chameleon?skin=profeb&lng=enMargaret's baptism appears in 1776