The inscription could be translated as:
Key (tablet) for the books of the Grand Emperess Dowager
It is noted in the Oriental Ceramic Society that this tablet possibly belonged to the grandmother of the Daoguang Emperor. It appears that plaques such as the present lot were used in the Imperial storerooms in the Palace to label the location of objects. They were attached to these objects by a cord, such as the example shown in The Forbidden City, Rotterdam, 1990, no. 76, p. 225, which is still tied to the original jade seal it is made for.
Compare also the example sold in our Hong Kong Rooms, 29 April 2002, lot 588; and another sold in these Rooms, 18 June 2002, lot 130