I have an ancestor named Mark Nelson born around 1770. He married Polly Allen, I've found information that they married in NH in 1794 and in Montpelier, Vt. in 1796. They had sons named James,1795, Joses,1797, Nathaniel abt.1813, and Alvah abt.1810. Daughters were (I think) Sally 10/1799, Mary 5/1803, Melinda, and Nancy. Mark and Polly are found in Woodbury, Vt. from 1810 on and also in Calais. (Lot 13J is on Nelson Pond which straddles the Woodbury/Calais line) If your info is correct this sounds like a younger brother of your James. I did find a James Nelson in Calais in 1840 who was between 70 and 80 yrs old, as well as a Peter Nelson. I'm afraid this isn't much help, I'm having trouble pinning down my gggg grandfather Mark before his marriage myself. In the 1850 census he is listed as being 80 and I think his birthplace is listed as Mass. but the handwrighting is not legible. Polly was most likely from Epsom, NH. I've probably not helped but if I can answer any qs email me, any info you can provide would be appreciated. I think we most likely have some kind of match here.