I
think you're right davem. It was my understanding that the two states of embodiment were first defined as unique in writings later than LotR, but now that I'm briefly flipping through
X: Morgoth's Ring I'm not so sure. There was a letter I read from the early '50s wherein Tolkien makes no distinction between the two degrees which led me to believe that he had not fully fleshed out the topic at that point, but he may only have been simplifying the explanation for his correspondent. Unfortunately,
Letters has a terrible index, so it will take me a while to find it again, and then to answer your question I will have to compare it with contemporaneous versions of the first chapters of the Sil. If nobody else pipes up with this information before I get around to it, I'll do what I can to find out. I'm sure Aiwendil or lindil could answer this off the tops of their respective heads.
All of my points/arguments in this thread relate to what I understand to be Tolkien's latest ideas on the subject. He was pretty careful to build
around LotR rather than in contradiction with it, and I'm sure he considered the later concepts compatible with those already in print. That said, he didn't leave us anything like my original post above. It may not be exactly how he imagined it, but he didn't tell us exactly how he imagined it. I have merely tried to draw a more complete picture.
Edit: Thanks for the research, Raynor.
This thread has a lot in common with my other threads and shares a lot of the same research, so I'll link to them here for reference:
Ëalar and Incarnation and
The Powers of the Istari.