From Dale Meegan:
It was an extremely wet and muddy morning's work but I made further progress on the cobbles. Tom Addyman found this 'lid' made of slate. Apparently these were used on the top of pots or jars. It was found in room 10, the southern of the two in the west range, now to be known as the larder!
Dale is an 'amateur' archaeologist working on the castle excavation.
Many thanks to her for picture and story.