No 4 rear sight with aperture milled off.

[Thunderbox]

There is a more prosaic reason for a battle sight being milled off on Parker's rifle (if the sight is original and not just someone's recent addition): it is perfectly possible to fit a target sight such as a PH5C and a standard Singer sight together on the same rifle.

Some people still do this so that you can easily switch from one to the other - i.e. you just remove the arm from the PH sight, and you can use the Singer sight. To do this, you do have to remove the battle sight, as it obstructs the eye piece mounting for the PH sight....