Putnam's original twin earth experiment shows that the meaning of "water" is determined by external factors. This is an epistemic fact about knowledge and meaning: What we really mean by using many of our words and sentences is not fully known to us, and is depended on the environment.
Yet, a lot - if not most - of the literature about the externalism/internalism controversy, describes externalism as a theory about the content of mental states, and hence as a theory in the philosophy of mind. According to it my own mental states are determined by external factors.
I find it very intuitive to believe that the meaning of our words is determined by the environment, but find it quite hard to believe that mental states are not internally individuated: Oscar and Twin Oscar are in the same metal state, yet, the meaning of their "water" is different. The two kinds of externalism, therefore, seem to me to be very different from each other. When and for what reasons was the original meaning externalism extended to be externalism about mental states? Are they really two different notions, or am I missing something?