Here is what I think of in regards to hooks- it's an offshoot of the rosette pattern, and could be over the spine in individuals with more aberrant patterns (less lateral symmetry).



I'm about to make myself look really stupid here, but hear me out. This picture (not mine) is a leopard that exhibits the hook as well as something like the keyhole that I had in mind. The keyhole I'm thinking of, however, was within a black dorsal stripe on a brown snake, not a continuation of the side pattern. Does what I'm saying make sense as far as what I'm referring to? I'm going to continue looking in the meantime.