The burial stone is in the Linsey Burial Ground, in Abbeville, SC. Note that the image of the stone at FindAGrave shows it to be a cenotaph, "In memory of…". Presumably someone erected the stone in the Lindsey Cemetery to commemorate Isaac. This may indicate either that that Isaac did not have an original stone in the cemetery, or that this cenotaph replaced the original stone, but it may also indicate that he is not buried here, and may not even have died in the area. Date of death given on the Cenotaph is 1831, so he could be the Isaac Cowan listed in Gwinett County, GA in the 1820 census.