Her age/inferred DOB seems to have varied a bit as between 1861 (29 b Northill), 1871 (35 b Liskeard) and her death in Liskeard in 1872 (aged "41"), but I would think she is likely to be the Betsy Ann Foster "21" b Northill who in 1851 is shown as a daughter of Richard Foster (53, gardener, b Northill) and his wife Mary (53 b Northill).
The 1851 census reference for the family (enumerated in Calstock) is HO107/1901/100/21. Using this reference alone should sidestep any transcription issues.