I'm not absolutely sure what this is but my best guess it is to clip onto a belt and attach a key ring to.

It has a recoiling cord about 3 feet (90 cm) long that retracts back into the fob. There is a small metal fitting on the cord that would take a standard keyring.

About 1 5/8" diameter (about 40 mm).

It's clearly old and battered with dents and paint loss but a great addition to any Marston's collection.