As far as I recall, M-400s started at serial # 100.
Here is an email I just received on my myspace site.
"Hey Frank,
Nice to know there's another Mellotronhead in the Old Dominion. I own a M400 s/n 121, which has a pension for playing radio station broadcasts louder then it's own tapes. One day I hope to speak with you about a restore of the old girl.
Jim -
Portsmouth, VA "
So this would be truly #21 then. That would be the oldest I am aware of.