Cerberus has a two headed brother, which made naming this defensive tool pretty easy.
I've been trying to design a single finger knuck that I like for years and finally came up with one. These are 1/2" brass. The finger hole is 1" and is elongated about 1/8" for your comfort and convenience.
I usually try to take photos outside to get the colors correct, so these photos aren't great since they were taken on my desk.