Abstract: A data card or display device has a display region of non-volatile, display medium, which is re-writable with humanly legible characters and an indexing track composed of a machine readable, coded pattern of indicia is marked along a datum edge thereof. A read-write terminal has a read head aligned with the indexing track and a write head which includes individual write elements extending laterally of a path of travel of the card or display device therethrough, thereby to erase and write pixels forming humanly legible characters at preselected longitudinal positions on the display region determined by indicia of the indexing track as detected by the read head. The pattern of indicia comprises two rows of identical, repeating indicia displaced by one quarter pitch.