The catalogue entry for the tape, dated 21st October 1965, reads:
Hot upward electronic arpeggios at rising pitches. (From TRW 6191 makeup).
This seems most likely to be Max Frisch's play The Fire Raisers, written in 1958: a "Brechtian fable: bourgeois household invaded by endlessly talkative terrorists intent on destroying it. Absurd comedy, or black parable of Europe in decline."