This part-tile comes from a very powerful tile inscription band with regular foliated hastae. In its original form, with the letters picked out in gold and outlined in red it must have been spectacular. One complete tile from this series appears to have been published (Carboni, Stefano and Masuya, Tomoko: Persian Tiles, exhibition catalogue, New York, 1993, no.5, p.10). There are a number (34) of fragments in the Victoria and Albert Museum, which include a date and signature (Lane, Arthur: A Guide to the Collection of Tiles, London, 1960, p.8, pl.5A). Further examples are in the Berlin Museum and in the British Museum (Porter, Ventia: Islamic Tiles, London, 1995, pl.28, p.41).
The date is certainly incorrect. It reads AH 206/821 AD which is clearly impossible. The V&A group is discussed by Dr Oliver Watson ("Fakes & Forgeries of Islamic Pottery" The V & A Album 4, 1985, pp. 39-43). Commenting on the group he notes: "There are other types of problematic object, but ones whose maker intended no deception. Such are straightforward copies after an original ... "There can have been no intention on the part of the potter or the architect to deceive his public, rather the inscription was a replacement during renovations of an original one in another material".