Quoth the Johns:
Those are human voices, but they were recorded one not at a time in the late sixties. The vocals are being played on a early version of a sampler called a Melotron. The British company was put out of business in the early '70s after a few short years when the UK musician union deemed they were losing jobs to the machine. My approach to this track was to create my own kind of HiFi test record, with the same kind of other-worldliness the originals seemed to be driving at.
[source: monopuff.org's online Unsupervised Libretto]