They didn't have the diary but had copies of two deeds that mention Seth. They did give permission for us to share the deeds in this blog. A transcription of the first deed is shown below.
BENJAMIN MATHES to SETH R SHACKFORD Book 109, Page 431
WARRANTY DEED DOVER
Land in Durham bounded:
Easterly by land of Dr. Ami R. Cutter and the said S. R. Shack-
Northerly by land belonging to the heirs of John Dame and
George Frost's land and land owned by the said S. R. Shackford to
Moses Edgerly Jr. land " and is bounded on the said Moses Edgerly
Jr. and and land belonging to the heirs of Capt. John Bickford and
Grasseswamp , so called, to the said Dr. Ami R. Cutters land first
Containing 24 1/2 acres more or less it being the same land I bought of
the heirs of Jonathan Langley.
Dated: July 1, 1818
Recorded April 6, 1821
Benjamin Mathes to Seth R Shackford, July 1, 1818, typed copy, , Warranty Deed; University of New Hampshire Special Collections & Archives, scanned copy.
Boardman Samuel Lane, 1836-1914, Family memorial: records of the Boreman, Bordman and Boardman families of New Hampshire and Maine (Augusta, Maine: no date given, before 1876), page 39-; digital image, Archive.org (https://archive.org : accessed 13 April 2015