Basics
For plating to be possible, there must be constant variants in details of the stamps printed from a single plate or lithographic stone so that one can identify the exact original position of each stamp. Evidence that may be used in plating includes defects or "flaws" occurring in the transfer of images, individual touch-ups by the engraver, recuttings of the plates, repairs, and accidental injuries to the plates. From their inception in 1840 until 1884, postage stamps from Great Britain incorporated control letters in the corners indicating the stamps's exact position on the sheet, e.g., A...C is first row, third stamp, and for a period also incorporated the number of the plate from which the stamps were printed. Other evidence such as color of the ink used or watermarks in the paper may be of use in reconstructing plates. In the absence of such evidence, plating is not possible. Because of the highly accurate methods of modern stamp production, most modern issues cannot be plated.
Read more about this topic: Plating (philately)