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They're not actually.
Brixton CC forum added them, and I asked the same question.
They all come from https://www.icons8.com and I've used only the Material design ones just to make sure they are consistent with each other. They are very close to what I did want designing, and I figure I'll see how it feels and if it feels good I'll go pay for the SVG versions to make them even nicer.
Are they bespoke David? I love the track icon.