Like many of the old posts regarding this couple, I too have been working on this lineage trying to connect Isaac Jett and Mary with their parents. I am a descendent of Isaac Jett Sr and Mary's daughter Nancy Jett who married Francis Gowin/Goins.
Is there any evidence other then the naming of grandchildren with the middle name of Letcher that Mary was a Letcher? I haven't found any - but I just want to make sure I haven't missed anything. I have spent a good deal of time researching the Letcher Family trying to find a connection to our Mary. It appears that most of the Letchers descend from a Giles Letcher that immigrated before 1740 to Richmond Virginia. One of Giles' sons named Stephen Giles Letcher 1740-1823 ended up in Madison, Kentucky. He had a daughter named Mary, but she was born in 1776 in Goochland County, VA. I have our Mary that married Isaac Jett was born abt 1765 so that doesn't look to be a match.
Stephen Giles Letcher had other brothers too as possible fathers for our Mary: William (VA), James (SC) and John (VA) but as far as I can tell their children were born later as well. BTW several Letcher descendants from this line went on to be Governors of VA, TN, TX.
We assume from the Kentucky County marriage records that Stephen Jett Jr is somehow related to Isaac's family - I have him as Isaac's brother as a placeholder. He gave approval for Nancy Jett to marry Francis Gowin on Mar 16, 1803. He stated that Nancy had lived in his household for a number of years and he was acting on behalf of her parents. In 1806, he attested to Marriage license permission given my Mary Jett for her daughter Haney (Henrietta) to marry James Mylar. It is interesting to me that Mary was not around to give permission in 1803 to the marriage of Nancy when I believe she was on the tax list in Madison in 1800 and was there from 1806-1830. Do people assume that Mary is a Jett related to Stephen because of Haney's permission document? I don't read it that way so I don't think that is the case.
I am going to research the Searcy's next because there seems to be a lot of documentation linking this Jett Family and the Searcy's. According to a document on Fold3, Isaac J Jett Jr testified in 1854 in Washington County, KY that he knew Rebecca Searcy, widow of Richard Searcy and was present the day she died in 1852. The Searcy's were from Granville, NC unlike the Letchers so its worth a shot. Also Isaac Jett Jr was living next to a Sam Searcy in 1820 in Madison.
Another wrinkle though is that Stephen Giles Letcher Jr also moved to Washington County, KY before 1830 from Garrard County. Hmmm...
Other unanswered questions for me:
Where does the Isaac's middle name of Israel come from? What source was used to determine that Mary was born in North Carolina? Her children all died before 1880, so the census records don't have that information. Is there a book with that info?