The marriage record of William Swain and Rebecca Williamson in Franklin, GA (bottom right corner). They got their marriage license on the 11th of March 1808 and were finally married on the 20th of Mar 1808. There is speculation that their marriage was recorded in 1808 but they were married earlier, but I don't think this is true. The Goodspeed History says that William married at age 26 which would make 1808 the correct year of marriage. This is another point to consider and helps solidify the argument that William Swain b 1800 and married to Rachel Anderson was not the son of this William Swain b 1782 since the latter wasn't married for another 8 years. Many genealogists also have a daughter Elizabeth Swain born before 1808 as a daughter of this union, but I think the relationship doesn't exist as she's not in the will of William W. Swain Sr. also.
"Georgia, County Marriages, 1785-1950," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/KXJ4-X33 : accessed 27 Jul 2014), William Swain and Rebecca Williamson, 20 Mar 1808; citing Franklin, Georgia, United States; FHL microfilm 7119.