This particular key is from a Thermaltake Poseidon ZX, but I've also seen it on a couple of others like the Rapoo V500. The ZX is one of those boards with weird stabilizers that left shift, caps lock, and enter won't fit. I fixed the enter/shift problem by just removing the stabs since they aren't even necessary. Only key I have left to get working is caps lock but it looks like it might be even harder to find than I thought.