It looks like the broken-off lid of an inkwell. It is approximately the right size, and the straight-ish edge looks like it has a couple of "broken" areas. Compare it with the various forms of brass lid for inkwells in the photos below.
It looks like the broken-off lid of an inkwell. It is approximately the right size, and the straight-ish edge looks like it has a couple of "broken" areas. Compare it with the various forms of brass lid for inkwells in the photos below.