It's the version outlined in red. It is quite clearly a north Somerset constituency; labelling it 'east' is absurd. The blue boundary, incidentally, shows the version of the seat as originally set out in the 1867 Reform Act, but it was switched to the version in red by the Parliamentary Boundaries Act 1868 and this was the form in which the constituency ultimately came into being. The blue (1867) version was even less easterly than the red (1868) version, but it was still called 'east'.
In 1885 boundaries were entirely redrawn and a new single-member seat of East Somerset was created, shown in light purple on a map here