This almost-complete finger ring with a projecting bit in the shape of a key is quite literally a key ring.
It consists of a simple hoop with an integral stem projecting at right-angles and terminating in a rectangular toothed bit, positioned so that it would sit along the back of the finger without getting in the way.
Despite the slightly cumbersome look, evidence suggests that these rings would have been worn on the finger. However, they were primarily intended as functional keys, rather than being a decorative item of jewellery.
Key rings of this form are thought to generally date to the 3rd or 4th century AD which fits well with the other artefactual material recovered from this context. They are common finds across the Roman empire and across rural, urban and military sites. They were generally used to open small boxes or caskets for personal possessions, rather than for doors or cupboards (Crummy 1983, 84-85).
Producing a key in a wearable form was a useful way of ensuring that your most important valuables were kept safe and secure and also meant that you were less likely to lose the key yourself. Important peace of mind when living in close quarters with many other people. Was this used by a soldier to keep his personal possessions safe whilst he was stationed at the fort?