I have records showing both Cathryn and Kathryn and also Dobbins and Dobyns. There are no Dobbins families listed on the 1790 census.
John Chapman, in The History of Edgefield County mentions an old ledger book with the oldest dated entry being 18 NOV 1771. It contains the following verse signed by Catherine D. Hammond. If this is the same Kathryn, then the entries are at least 6 years older than the receipt date, as this Kathryn died in 1765.
Cathryn's granddaughter, daughter of Samuel, was also named Catherine Dobyns. She was born 30 DEC 1763 and died 11 MAY 1842. The nature of these verses seem too old for the younger Catherine. I believe they should be attributed to the elder grandmother. The oldest receipt is for 27 pounds, and bears date 18th November, 1771; the latest bears date May 2nd, 1782, and is for the sum of nine pounds, six shillings, and eight pence, Sterling; paid by Richard Guinn to John Dawson, in full for the balance of ninety bushels of corn and a barrel of rice.
