'Also there is ONE Susan St. Clair... died 30 aug 1877 age: 50y, note says w/o J St. Clair.'
It is assumed that this is Susan St. Clair, widow of James St. Clair, and that her age should have been 40, not 50. Her daughters are in the 1880 census with their aunt and uncle, and her son is a servant in 1880, so it is assumed she died before then.