Perhaps it was that when he listened back to the recordings of instruments played back, the higher notes just sounded too weird. I still believe that the 35 note range was the only useful one. Perhaps he could have gone a bit lower on some instruments and higher on others, but overall, the 35 we have now was a compromise. Even now, boys choir does not sound like anything of this earth in the higher registers. How far up could you go with flute? Or Down? 8 choir is already doubled in the right end. Obviously the Bradleys agreed.
Frank
Having a good working knowledge of all wind and string instruments through my 23 years of teaching in schools, and having recorded more than my share of mellotron tracks I can say definitely that most instruments have a useable range of less than 35 notes. If you get up into the extreme high range, they just don't sound all that good and it can be REALLY hard to hold some of those notes rock steady for the required 8 seconds. Keyboards and strings are an exception to this, but 35 really does give you the best part of most instruments. Voice is the big exception here. There are not too many singers who can cover the needs of a mellotron keyboard all by themselves -- and I wouldn't want them to try!
Personally, I think Harry came up with the actual pitches because of the violin, arguably the most famous of all sounds, whether Chamberlin or Mellotron. The lowest note on a violin is low G and the F at the top gives you quite an adequate range.
That's my story, and I'm sticking to it.
Rick