Mellotronists

Certainly the M300 was a transitional design, but I can just imagine 
a little staff meeting at the factory after the initial run of 
Mk2's. Someone of hierarchy saying "Okay, gang--these pricey 
monstrosities are a bit too much to manufacture--the mechanics, the 
cabinetry...the delicate inner workings, everything...and they 
aren't selling too hot, so we need to scale these things down and 
simplify them a bit & keep 'em unique at the same time."

That, perhaps weighed against musicians suggesting a more 
portable/roadworthy unit.