1971 MELLOTRON M400S SN 348

via this auction where you'll find tons of pics including more of the inside.

"For your consideration is this vintage original 1971 Mellotron M400S keyboard by Mellotronics Limited. Favored by many musical giants from the 1960s to the present, the Mellotron is the signature sound of countless hit songs. The M400 model, the iconic white Mellotron, is still quite rare. Though they were manufactured from 1970-78, there were only about 1800 made, and significantly less are still working, or even in existence. This particular example, serial number 348, is an extremely early Mellotronics-manufactured example in complete condition with he original expression/volume pedal and the original Bulgin power cable! Take advantage of this very reasonable Buy it Now/Best Offer listing, and become the proud owner of a magnificent piece of history!

This example has an SMS-3 circuit board, while Mellotrons from this era are usually equipped with a CMC-10 board. It is possible that this 'Tron was serviced sometime in the mid-70s and that the circuit board was replaced, as the CMC-10 boards are very unreliable and were known to slow the motor down causing a change in pitch.

Cosmetically, this 'Tron has lots of authentic vibe. While it's not in perfect shape, it is in excellent condition with the usual weather-checking, faded paint and finish scuffs. The overall appearance is quite handsome with no significant condition flaws, no modifications, and no repairs. All of the components display matching serial numbers (348) AND matching cabinet numbers, (131). When they were being manufactured, Mellotron M400s keyboards carried a retail price in the area of $5,200... ...in 1973, which is approximately $25,000 in 2010. Yikes. With seemingly timeless musical applications, this unique sounding keyboard with beautiful eccentricities and idiosyncratic behaviors, has yet to be accurately reproduced with even most advanced sampling technologies.

This particular example was removed form storage, and has not been used within the last 30 years. The unit is missing one fuse cap, and has consequently not been turned on. It is being sold in as-is (assumed) non-working condition as a unit that needs to be completely serviced. It does not include a tape rack."