Well, I went through the trouble of finding a cassette tape to show my kids how the GU unit worked. Used it for a couple of months until they lost interest on it and only then replaced.
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I do still have somewhere within all my crap 2 x 10 packs of TDK 90min Cassettes. I am sure if I go through my boxes of vinyls (yes I still have my old records but no way to play them) that I will find some old original and recorded tapes.