The gourd shape works at this size because the profile is unmistakable even at two and a half centimetres — a narrow waist between two bulbs. Solid brass, cast in a single mould with the eye formed as part of the piece rather than welded on.
A small percentage of castings lose their ring through wear if the metal is thin at the eye. This one is cast thick there, which is the part worth checking on any brass keyring.