I found it 2ft under the stone floor of a derelict building in my garden while I was laying a cable. It was a lump of soil whose shape looked suspicious. It tinkled as I lifted it. How long, I wonder, since that sound had been heard? I believe the design has remained pretty much the same since neolithic times. The building has probably been there since around 1400, so I imagined it to be considerably older than that. Perhaps neolithic. Perhaps Roman (there are stone drains close by, possibly Roman). It does have quite a lot of encrusting.
But I came across another just like it (see the web site below**) which is described as being a 19th century horse harness bell. Perhaps the floor of the building was much more recent than the walls. There are tethering niches in the walls, so it seems likely that this was for a horse.
**PS I don't approve of metal detecting, but the site did have some interesting things.