I looked at the 1880 census again on Stephen and Abigail and happened to notice their ages. Stephen was 67 and Abigail was 30. Their children were 14, 9 and 3. That means Abigail was only 16 when her first child was born and Stephen was 53. The ages are correct, on the census, his is re-written beside the other one and all 6's are written the same. This doesn't seem like a likely match, even in those days. I am wondering if this is a 2nd marriage for Thomas.
I can't find the 1870 census for them, either. I hope someone has a likely solution.