John's Mellotron, acquired in 1965, and which originally sat on the half-landing leading up to the attic (see Joe Baiardi's Kenwood video, where he passes the very spot at 5:48), is familiar from his home demos (Pedro the Fisherman et al). The amusing film on the left features a Mark II model, of the type that John had, and similar to the one used on Strawberry Fields. (That wasn't John's Kenwood Mellotron, however, but another hired Mark II; EMI eventually bought their own in 1968, the slightly different "FX" model.) Only around 2-300 of the Mark IIs were ever manufactured, mainly because of the huge cost - in modern money, roughly £13,000 each. Anyone who has wasted half an hour listening to John's attic noodlings will instantly recognise the Mellotron stylings of the two gents in this film (and if it doesn't, at least, raise a smile, then seek medical attention).
Finally, here is a page from the Mark II operating manual, explaining just what all those knobs and switches do:
From this, we can deduce that in the first pic above, John was captured varying the pitch up (or down), meaning that he could "strike the same notes and play in a different key!"
Enthralling stuff.
(By the way, notice that one of John's Rhythm Fill knobs seems to have fallen off...what is it with John and his knobs? As anyone familiar with Andy Babiuk's rather wonderful Beatles' Gear book will know, John also had a problem keeping the knobs on his Rickenbacker 325.) (That's enough about John's knobs - ed.)