You mean you haven't got any?!!
I remember there was a bit of talk back in the day about the possibility of SQ encoded cassettes but it obviously never came to anything commercially. The concern always seemed to be that the phase response of cassettes wasn't good enough. In fact it would appear that the performance of run-of-the-mill domestic reel-to-reel decks of the day wouldn't be good enough either. Sony made a bit of a thing about a range of reel-to-reel decks they branded as "Uni-Phase" being suitable for the purpose - here's a bit of blurb on one such:
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Interesting that the 'cure' simply seemed to involve a decent set of heads! Many cassette decks would go on to have very exotic head designs and with the dual capstan drive most hifi decks then adopted I don't think head contact would have been an issue for SQ playback. Perhaps that technology (like decent decoders) just came too late. Although I half wonder if the problem might at least at much have been at the duplicating end where in the early days cassettes were mass produced on some very ropey equipment!
Actually there were quite a number of quad matrix encoded cassettes .
The BASF SQ cassettes were emblazoned with the SQ emblems and sticker so were hard not to miss.
Vox had a number of QS encoded cassettes indicated as Quadraphonic.
Angel had the double circle SQ encoded cassettes and 8 track.
Peters International had cassettes fr French Decca.
Radio Shack had a few EV -S4 encoded .
Both EMI and Columbia had encoded cassettes not indicated as such. Joe Dash at CBS confirmed that all Columbia Masterworks and O .C. Broadway Cast single inventory that had vinyl counterparts are indeed encoded.
Fosgate had 2 demo cassettes of stereo and SQ tracks , one of which I had with my Tate decoder.
There were a handful of others , but for the most part if it had a single inventory matrix encoded album , the tape would be quad encoded as well.