The novel method comprises (1) producing in one major surface of a metal masking plate an array of substantially-parallel grooves separated by ridges of plate metal, (2) producing in the other of said major surfaces an array of generally rectangular-shaped depressions opposite the grooves and extending only partially through the plate and less than the distances required to connect to said grooves, (3) filling the grooves with an electrically-insulating material, (4) removing the ridges of metal down to depths to connect with the depressions, thereby producing an array of substantially-rectangular apertures through the plate and electrically-insulating strips across the apertures and (5) covering selected surface portions of the electrically-insulating material with an electrically-conducting material. Before step (3), the surface of the grooves are coated with a resistive or semiconductive material, such as black iron oxide.