You will find cutts in the salt though that have no line at all under there jaw, very common salt coloration, the "yellow" slashed ones you saw where more than likely just starting to color up.
I dont know too much about the whole maxillary jaw theory, i have really only noticed that difference to be the deciding factor on male vs. female.
I think you just hooked into a REALLY nice rainbow. Or even a small steelie, though with the body type/structure i dont think so. There are tons of really nice fish in the canal rivers from the skok all the way up to the quil. could be a fish just summering in the canal because of better habitat...meaning: temps, food, cover, which we know sometimes the canal rivers struggle with. Either way Killer rainbow though, i hope He spawns to make more girthy, plump breathren.