As the field from which this came - actually it used to be the village green - had yielded nothing earlier than Georgian material (1730 - 1820-ish) I assumed this, at first, to be a fairly recent, possibly very recent, object. I was amazed to have it identified by Dorset's FLO (Finds Liaison Officer) as being from the 1300s.

To hear the (extremely stupid) story of its discovery, read Stupid Brothers no. 4

Originally it held a small, slightly concave glass mirror, which would have been one of the very earliest ground glass lenses ever made in England. It therefore represents the first time that people could check up on their appearance, accurately, wherever and whenever they wanted - a huge leap forward for our collective vanity. It's extremely rare to find one of these intact, as the hinge is very small and therefore very fragile indeed.