That's an interesting piece for sure. I really have no idea what it was made for though. I thought at first it was on a grenade pin ring, but they were made of round wire not flattened like yours. It's one of those things that there have probably only ever been one of, and made for someones personal reason. It may be impossible to find out without the maker around to tell the story.
If I had to hazard a guess, and this is just a WILD guess, I'd say it could be a scrap of metal from the war that a soldier picked up maybe from a battlefield, as a sounenir and labeled then used it as a key chain later. Why he chose that scrap, what it's from, and where it came from could likely always be a mystery though.
