Haven't seen this exact ring, but in the 70s they sold similar style rings that you wore one on each of two or all four fingers, when a fight was brewing, you rolled the flats against each other and you basically had brass knuckles. The style I've seen had bigger knobs than this one. There was a legal loophole, since they were not connected they were classed as "jewelry" not brass knuckles or knuckle dusters. Like the style of yours though!! Nice find!!
Almost forgot! They sold them in Tijuana. Heck they might still be selling them or similar.