There's no detail on it. It's just smooth. The back side is flat while the part showing is rounded on the edges. It appears to be copper rather than brass. It sounded off as zinc.
Here's What I am talking about. Only the ones pictured have the snap prong in them. The "eye" has a spring wire inside that captures the snap prong even though the hole is irregular shaped. (Hard to explain).
Monty
I can't find a pic but it looks like a washer or spacer that fits over a shaft or impeller. The closest thing I can think of off the top of my head is a fishing reel crank. Yours is a little big for that but thats what it reminds me of.
I think its just an anti-rotation or non-turn washer/spacer - slipped over a shaft that has one (or two) flattened sides. Shown are bicycle parts but you get the idea. If it is copper, it could be for plumbing...