Might be worth getting the birth cert for John Salter issued Sept Qtr 1839 giving his mothers maiden name as Connett. There might be a connection somewhere, e.g. it might be Susans maiden name. The Salter family were in London before going to Topsham, so she would not necessarily be with them then.
My great grandmother registered one of her sons with her name, (not married AT ALL)and used her aunts surname for her maiden name, am guessing she was at her aunts for the birth anyway. People could get up to all sorts then, no way of checking.