They are marking the Christians’ homes with ن, a nūn, for Nasara, which is the Qur’an’s word for Christians: Nazarenes. Probably they are doing this so that they can know from which houses to demand the jizya, the tax that the Qur’an demands that the subjugated People of the Book pay in the Islamic state: “Fight those who believe not in Allah nor the Last Day, nor hold that forbidden which hath been forbidden by Allah and His Messenger, nor acknowledge the religion of Truth, (even if they are) of the People of the Book, until they pay the Jizya with willing submission, and feel themselves subdued” (Qur’an 9:29).
Images from R (thanks to Jerk Chicken).