Rumor has it that the very first cassette single, Bow Wow Wow's C30, C60, C90, Go! from 1980 -- a song that is about home taping -- also had a blank B-side for the same reason, but I can't find any photographic evidence of this.
That's totally the kind of prank McLaren would have pulled, though. It's also totally the kind of thing he would have taken credit for after it happened by accident.
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I'm just gonna leave that here.