Interesting ...
"A cottage industry developed in the trenches during the war, making souvenir aluminium rings. Aluminium rings were made from melted down fuses from German shells, or from melted down aluminium mess kits and canteens. Collecting the fuses from unexploded ‘duds’ could often be dangerous and many men were wounded attempting to salvage this material. The aluminium would be melted, then roughly cast and filed down, and then either be engraved, or have an item, such as the front of a button, attached to the face of the ring"
Like this one - not exactly like yours.
https://www.awm.gov.au/collection/REL/13693.001