Found this Hugh Hopper interview on Mark Hewins´ Vimeo. It does not say when it was done.
"I asked Hugh about his methodology, concepts and process in regards to tape looping in the early 1960's.
We especially prepared the Music on magnetic tape for this film, which Hugh starts at the beginning; it plays constantly underneath the dialogue.
(Mark Hewins, Processed Synth Guitar - Hugh Hopper, Bass/Fuzz Bass)".