It's an offshoot of the Mellotron[1] which was basically the original sampler. The mellotron was/is a keyboard where pressing each key spooled out and played back an individual tape strip of a real string section, cello, flute etc. It's the string sound that's really famous and recognisable. The mechanisms make a VHS look simple, see the wiki article for diagrams.

The linked machine is a read-write version of the mellotron, created especially for the BBC to do sound effects for radio programmes. One of the many one-of-a-kind cutting edge devices commissioned by the BBC in the glory days.

[1] https://en.wikipedia.org/wiki/Mellotron