The letter is written on a Robert Fraser Gallery letterhead and dated september 5, 1969.
The handwriting changes quite a lot between the beginning and the end of the letter (in attached files you'll find 3 samples of this letter, one from the beginning, one from the center and one from the end). I know Lennon was trying to stop his addiction to heroin a this very moment (perhaps it can be an explanation?).
The signature is also very unusual.