In ROTS, we see Vader's helmet assembled as two separate pieces, however I have noticed that in ESB in the meditation chamber, if you pause at the correct moment, you can see that his helmet is assembled as one piece.
Could this suggest that as he got more accustomed to the suit or the assembly technology got more advanced, the helmet was put on in one piece rather than two?
There is also an extra bit of evidence that has been revealed in the latest Kenobi episode, however I won't post it here due to spoilers, and I have commented in the Kenobi thread with the extended version.